The text about "German historical school has promoted and spread a theory that Slavs settled in the Balkans in the 6th century, meant to undermine the Serb presence and significance in the area during the Roman era" is a conspiracy theory, and it would be pretty hard to find sources (other than perhaps a couple from Belgrade) which support such rants. Linguistic evidence (I could cite a reputable source here but the name alone would spark more drama ;-) hints at Albanian-Slav contact before the end of the ninth century - when there was a vowel shift in local Slavic language. Some loanwords in Albanian were taken before that vowel shift. However, "Albanians"  ≠ "Kosovo" and "Slavs" ≠ "Serbs", of course. However, all this is relevant to many different articles (because the history of the region is told over and over again, in different ways, on different pages) so we should probably discuss it somewhere more central if we're planning to make serious improvements.