The Balaton principality under Kocel was at this time probably a vassal of East Francia, my suggestion then would be to put both principalities (Balaton and Pannonian Croatia) under Frankish rule and make their historical rulers as vassals of East Francia and give them their fiefs which they controled in reality, its been similarily done this way in vanilla VC with Aethelred and Alfred of Wessex as "co-rulers" although Alfred was just a normal vassal (for example Kocel would be given the fief of Blatnohrad and perhaps some nearby village, he would recruit Slavic units from his fiefs and they would all follow Christianity as he did in reality). Another reason to do this might be that we know of practically no nobles from this time which would be vassals of Kocel or the other Croatian duke, so putting them under Francia would make them somewhat stronger and you wont get small empty factions with no meaning.
Actually it fell under Moravian rule until about 10 years later so I think keeping it the way I proposed would make more sense. Now that you mentioned it, at this time a duchy of Bohemia was created ruled by Borivoj, although it seems it still was a part of Moravia and both Moravian and Bohemian rulers were good allies, so again I would put Borivoj as vassal of Svatopluk and give him Prague and nearby villages as a fief and give him the title Duke to reflect his position.
I also think you should wait some time until you get enough progress to show to the community