To make things more simple:
Croats, Serbs, Bosnians = same people
Macedonians = floatant slavic mass, basically mix of Serbs and Bulgarians
Albanians = chechenya/dagestan thinking they are Illyrians*
Bulgarians = Turkic tribe + slavs
Greeks = Greeks
There are in fact a lot of suggestions that they came from present day chechenya/dagestan to balkans due to preassure from mongols/turks. In fact, first records of them from around 12th century state that there is around 100 families that live in the mountians and they grow cattle. Further more, there is an area of chechenya called Albania, and a lot of cities in Albania don't have names that originate in Albanian language, yet, they claim to be illyrian.
You can spin these things any way you want to serve your purpose. A nation is defined as a group of people with common history and sense of identity. As someone already pointed out: Albania didn't exist as a state in 1444, a large chunk of it wasn't even inhabited by Albanians, Skenderbeg himself had a Serbian mother, and his family, Kastrioti, were in fact Greeks.
*cough* Caucasian Albanian Language doesn't have anything to do with Albanian Language/People, not even Indo-European People. They are related to the Dagestanis and Chechens *cough*