Now some excerpts about Slavic invasion of the Balkans (they did not move in as "peaceful farmers", contrary to what some users claimed):
Procopius, Book V, XXVII, 134:
"(...) This exploit, then, was accomplished by the Goths on the third day after they were repulsed in the assault on the wall. But twenty days after the city and harbor of Portus were captured,
Martinus and Valerian arrived, bringing with them sixteen hundred horsemen, the most of whom were Huns and Sclaveni and Antae, who [Sclaveni and Antae] are settled above the Ister River not far from its banks. (...)"
Procopius about Slavic invaders capturing and enslavic a lot of Romans:
Procopius, Book VII, XIII - describing events in year 545 AD:
"(...) For
a great throng of the barbarians, the Sclaveni, had, as it happened, recently crossed the Ister, plundering the adjoining country and enslaved a very great number of Romans. (...)"
Procopius of Caesarea:
"(...) In Illyria and Thracia, from the Ionian Gulf to Byzantine surrounding cities, where Hellas and Chersonese regions are situated, (...) the Sklavenes and the Antes, penetrating practically every year since Justinian administering the Roman Empire, were inflicting irreversible damage to their inhabitants. In each invasion I estimate 200,000 Romans were either took as prisoners or killed (...)"
Procopius about Roman attempts to stop the Slavic invasion:
"(...) the Empire wasn't able to find just one only man just as brave to undertake this task. (...)"
Pope Gregory I in a letter to Exarch of Italy from year 599:
"(...) It deeply afflicts and disquiets me
the Slavic nation that menace us. It afflicts me from what I already suffer from you
, it disquiets me because they have already started to penetrate into the Italic peninsula through Istria.(...)"
And according to Priscus of Panium, in 610 Slavic tribes flooded into Greece.
Procopius of Caesarea:
"(...)
Nay further, they [the Slavs] do not differ at all from one another in appearance. For they are all exceptionally tall and stalwart men, while their bodies and hair are neither very fair or blond, nor indeed do they incline entirely to the dark type (...)"
Procopius of Caesarea:
"(...)
In more or less the same time [549 - 550] a Slavic army (...) gathered itself together and after crossing without encountering any resistance from anyone the river Ister [Danube], and later with similar ease the river Heuros, it divided itself for two parts. (...) Commanders of Roman garrisons in Illyria and Thrace fought against both those parts and even though they had already separated from each other, the Romans suffered - contrary to their expectations - a defeat, and some of them fell dead on the spot, while others found salvation in escaping. (...) After all garrisons had suffered such defeats at the hands of either one or the other one of barbarian armies, one of enemy bands fought against troops of Asbados. He was a member of Emperor Justinian's personal guard (...) and he led a numerous and elite force of cavalry, which had been garrisoned for a long time inside the Thracian stronghold of Tdzurulon. But also they were forced to retreat by the Slavs and most of them, shamefully escaping, got slaughtered, while Asbados himself was captured and temporarily left alive, but soon after that the Slavs skinned him alive and threw him into a burning campfire.
After that the Slavs were plundering all neighbouring Thracian and Illyrian lands without any obstacles and both of their two units captured many strongholds. (...) And those who had defeated Asbados, later
plundered in turn everything up to the sea coast, and captured in an assault the coastal city of Toperus (...) And they slaughtered 25,000 men, plundered everything, and enslaved all the children and all the women. (...)"
John of Ephesus:
"(...)
In third year after the death of Emperor Justin, during the reign of victorious Tiberius, the damned nation of the Slavs has risen, and marching through entire Hellas, through lands of Thessaly and Thrace, captured many cities and strongholds, plundered, burned and robbed, seized the land and settled there with full ease, without fear, like in their own land. (...) they were plundering the country, burning it and robbing, as far as the Great Walls [of Constantinople], and this is how they captured many thousands of cattle, as well as many other kinds of booty. (...) Until today, that is until year 584, they still continue to live in peace in lands of the Rhomaioi, without fear and concern, plundering, murdering and burning, getting rich and highjacking gold and silver, capturing horses and plenty of weapons; and they have learned to fight better than the Rhomaioi. (...)"
Menander Protector:
"(...) About the fourth year of the reign of Caesar Tiberius Constantine, some hundred thousand Slavs broke into Thrace, and pillaged that and many other regions. As Greece was being laid waste by the Slavs, with trouble liable to flare up anywhere, and as Tiberius had at his disposal by no means sufficient forces, he sent a delegation to the Khagan of the Avars. (...)"
Strategikon of Maurice:
"(...) They [Slavs] do not keep prisoners in perpetual slavery like other peoples, but they demarcate for them a limited period of time, after which they give them a choice: they can return home after purchasing their freedom, or stay among them as free people and friends. (...)"
Jordanes:
"(...) These people, as we started to say at the beginning of our account or catalogue of nations, though off-shoots from one stock, have now three names, that is, Venedi, Antes and Sclaveni. (...) they now rage in war far and wide, in punishment for our sins (...) Though their names are now dispersed amid various clans and places, yet they are chiefly called Sclaveni and Antes. (...)"
Procopius of Caesarea (about squatting Slavs):
"(...) Belisarius was eager to capture alive one of the men of note among the enemy, in order that he might learn what the reason might be why the barbarians were holding out in their desperate situation. And Valerian promised readily to perform such a service for him. For there were some men in his command, he said, from
the nation of the Sklaveni, who are accustomed to conceal themselves behind a small rock or any bush which may happen to be near and pounce upon an enemy. In fact, they are constantly practising this in their native haunts along the river Ister, both on the Romans and on the barbarians as well. (...)"
Abraham ben Jacob (a 10th century Sephardic Jewish traveller from Muslim Spain):
"(...) Slavic countries extend from the Mediterranean Sea to the Northern Ocean. (...) Generally speaking, Slavs are warlike and violent, and if not their internal discord and lack of unity, no other nation would be able to match them in strength. (...)"
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And here about the arrival of the Croats (but it was much later - not during the 500s, but during the 600s):
Constantine Porphyrogennetos, "De Administrando Imperio":
"(...) their ancestors were Pagan Croats and Serbs, known also as White [Croats and Serbs]. Great Croatia, called also White [Croatia], until today is still Pagan, just like neighbouring [Lusatian / West Slavic] Serbs [Sorbs] (...)"
And another excerpt - "De Administrando Imperio":
"(...) Therefore everyone, who would like to do research about Dalmatia, can read herein about the way how the Slavic peoples took it. The Croats with their families came to Dalmatia and found the Avars in control of that land. After fighting against each other for some time, the Croats defeated the Avars, partially murdered them and partially forced them to submissiveness. Since that moment the country was seized by the Croats. (...)"
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However, I would agree with you that large part of ancestry of South Slavs comes from Pre-Slavic inhabitants.
This seems to be confirmed by the way how Slavs treated captives and prisoners of war - they often allowed them to join their communities.
As
Strategikon of Maurice wrote:
"(...) They [Slavs] do not keep prisoners in perpetual slavery like other peoples, but they demarcate for them a limited period of time, after which they give them a choice: they can return home after purchasing their freedom, or stay among them as free people and friends. (...)"
And now let's quote "Phenotype of old Slavs, 6th to 10th centuries", by Łukasz M. Stanaszek:
http://www.archeo.uw.edu.pl/swarch/Swiatowit-r2001-t3_(44)-nB-s205-212.pdf
"Phenotype of old Slavs, 6th to 10th centuries", by Łukasz M. Stanaszek.
I. Byzantine sources:
1. Procopius of Caesarea (6th century):
- "(...) Valerian chose one of the Sklaveni who are men of mighty stature. (...)"
- "(...) Nay further, they do not differ at all from one another in appearance. For they are all exceptionally tall and stalwart men, while their bodies and hair are neither very fair or very blonde, nor indeed do they incline entirely to the dark type, but they are slightly ruddy in color. (...)"
2. Theophilact Simokatta (describing events from year 595):
"(...) The Emperor was with great curiosity listening to stories about this tribe, he has welcomed these newcomers from the land of barbarians, and after being amazed by their height and mighty stature, he sent these men to Heraclea. (...)"
3. Theophanes the Confessor (describing the same event from year 595):
"(...) The Emperor was admiring their beauty and their stalwart stature. (...)"
4. Pseudo-Maurice (Strategikon) - late 6th century / early 7th century:
"(...) Tribes of Sclaveni and Antes (...) are very resistant to hardships, they easily endure both heat and cold, rain and lack of garment. (...)"
5. Constantine Porphyrogennetos, "De Administrando Imperio":
About ancestors of Balkan Croats and ancestors of Balkan Serbs:
"(...) their ancestors were Pagan Croats and Serbs, known also as White [Croats and Serbs]. Great Croatia, called also White [Croatia], until today is still Pagan, just like neighbouring [Lusatian / West Slavic] Serbs [Sorbs] (...)"
II. Muslim and Sephardi Jewish sources:
6. Al-Baladuri (late 7th century):
"(...) Slavs are a tribe of ruddy complexion and fair hair. (...)"
7. Ibn Qutajba (describing events from years 691 - 694):
"(...) If only Prince wanted, outside of his doors would be black Sudanians or ruddy Slavs (...)"
The same information is repeated also by 8. Al-Baladuri (9th century).
9. Al-Ahtal (late 7th century):
"(...) Birds of the desert saw in those people a crowd of fair-haired Slavs. (...)"
10. Jaqut (13th century, but using much older primary sources):
"(...) Slavs are a tribe of ruddy complexion and fair hair. (...)"
11. Ibn Al-Kalbi (late 8th century / early 9th century):
"(...) Slavs are a numerous nation, fair-haired and of ruddy [pink] complexion. (...)"
12. Al-Gahiz (early 9th century):
"(...) Among Slavs, abominable and ugly are their smoothness of hair [as opposed to curly hair] and delicateness, as well as blond or ruddy colour of their hair and beards, and also whiteness [bright blond colour] of their eyelashes (...)"
Al-Gahiz:
"(...) Tell me friend, after how many generations a Zang became black, and a Slav became white? (...)"
13. Abraham ben Jacob (years 965 - 966):
"(...) What is peculiar [when it comes to Slavs], most of Bojema people [Bohemians / Czechs] are of swarthy complexion and dark hair, while fair colors are rare among them [compared to frequencies among other West and East Slavs]. (...)"
Abraham ben Jacob:
"(...) Slavic people are often haunted by two diseases (...) these are two types of rash: redness and abscess (...)"
14. Ibn Al-Faqih (10th century):
"(...) There exist two kinds of Slavic people. First kind are people of swarthy complexion and dark hair. They live near the coast of the [Mediterranean] Sea. The other kind are fair people, who live inland. (...)"
Ibn Al-Faqih:
"(...) Inhabitants of Iraq are people (...) who are not born with hair colour intermediate between russet, blond, matt-white and white [bright blond], as it happens among children born from Slavic women. (...) People of Iraq are free from russet hair colour of Slavs. (...)"
15. Al-Masudi (10th century):
He writes that dominant complexion among Slavic people is fair, not dark.
III. German sources:
15. Saint Bruno of Querfurt:
He wrote that in Poland having a beard is a common custom among men.
http://s4.postimg.org/i64k6ay25/Slavs.png
However, there are considerable differences in pigmentation between various South Slavs.
The most dark-pigmented are probably Bulgarians - apparently the most strongly hybridised ones.
Later during times of the Bulgar Empire, Bulgars were also settling Byzantine prisoners in their lands.
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I do not exclude the possibility, that some of South Slavs mixed with Goths (who had been there before Slavs came).
However - not much seems to indicate, that this particular haplogroup (I2a-Din) is Gothic. It seems more Slavic.
Mayber both Slavs and Goths had that haplogroup already before coming to the Balkans ???
Let's wait for ancient DNA.
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As for the Avars:
Menander Protector:
"(...) About the fourth year of the reign of Caesar Tiberius Constantine, some hundred thousand Slavs [but 100,000 of just warriors, or men+women+children?] broke into Thrace, and pillaged that and many other regions. As Greece was being laid waste by the Slavs, with trouble liable to flare up anywhere, and as Tiberius had at his disposal by no means sufficient forces, he sent a delegation to the Khagan of the Avars [to ask them for help against the Slavs!]. (...)"
Indeed, the Avars initially became allies of the Romans against the Slavs! Only later - after defeating (and subjugating) some Slavic tribes, and negotiating peace with some other ones - the Avars turned against their former allies (the Romans) and - together with the Slavs - started attacking the Empire.
Moreover, the combined Avar-Slavic (or rather Slavic-Avar) army, often commited very nasty things - including expulsions of local populations.
The Avars all fought as horsemen (they had no native Avar infantry!) - they would move in front of the advancing invasion force, plunder settlements, remove the local population, and Slavs (mostly infantry, but some horsemen too) followed with their families, and settled the deserted ("cleansed") land.
This is also an answer to question asked by Maciamo "what happened to local pre-Slavic inhabitants?". In some regions, the locals were assimilated by Slavs. But in some other regions, the area was ethnically cleansed by Avars (with help of Slavs), and Slavs settled empty territory. Maybe founder effects took place in such areas (it depends whether the number of colonists in those deserted areas was large - it could be small and not genetically average / representative).
Just look what Yugoslavians did to each other - war, genocides, religious hatred... 
