I can say something about the slovene ancestors only. We're pretty sure (based on the dates of extinguishment of late roman episcopal centres on our territory, and also based on recently aquired archeological data) that the Slavs settled in modern day Slovenia more or less in the second half of the 6th century. This was allowed by the migration of the Langobards from the eastern Alps into northern Italy, that happened in 568. After that date the Avars took control of the territory, they already ruled the Pannonian plain slighly before that. The Avars were basically just a central asian military elite and not a numerous people. The Slavs became their subjects and the Avars allowed their eastward migration. The sources mention the Slavs in 592 and 595 while they were fighting with the Bavarians in the upper Drava valley (today in central Austria - eastern Tyrol ), which means that they aleady setled most of the eastern Alps before that. So basically the slavic settlement of the eastern Alps can be dated between 568 and 592. From which direction they did come, is whole another question. It is generally beleived that modern day Slovenia was setlled from two distinct directions. An earlier (soon after 568) is considered to have come from the North (ethnically this is a west slavic element). The migration path probably coincided with the roman road that led from Vinodbona, passing trough Scrabantia and Savaria towards the south. In the slovenian region of Prekmurje there is the richest archeological evidence of early Slav settlements, which can probably be atrributed to this wave. The second wave came from the Balkans (south slavic ethnic element), and is considered to have happened about 580-590-ish. So this can be considered the end of the slavic settlement in the Balkans.

About the second question, who was here before the Slavs? The general name with which the slovene historiography refers to the previous inhabitants is "staroselci", meaning "previous settlers". It is generally beleived those were mostly romanized indigenous peoples, that retained some of the cultural traits from the pre-romanized era, that is, from the late iron age cultures (La Tene period), which are mostly associated with Celtic and Illyrian ethnicities (although we today know that the Illyrians were by no means a culturally homogenous group). We have several names of the tribes, and also 4 main archeological cultures clearly defined over the territory of modern day Slovenia. Those peoples retained their living style in hilltop fortified settlements also after being heavly influenced by roman culture after the 1st century AD. The romans settled the plains, where they developed agriculture, settled their war veterans and instituted cities (Emona, Poetoviona, Celeia etc) which were important traffic nodes. So the old populations (I guess especially the urban ones that were most exposed to germanic incursions) moved towards the Adriatic coast and towards Italy. The settlers in the hills remained also after the slavic settlement and they were mostly called "Lahi" by the Slavs. Many placenames derive from this term (most notably the town of Laško). Their communities surely persisted during the middle ages, but they were eventually assimilated into slavic culture at some point.