The free soldiers stuff is far too OP atm. I was playing in Mali, and managed, after much skin-of-my-teeth gameplay, to rise from a count to forming the kingdom in 3 generations. A few years in, an independence faction starts with 2 members (my only 2 vassals), their power is ~10%. I could raise just over 4k levies not including the factions, plus after shifting my capital had access to 4800 mercs, for a combined army of 9k. The faction revolts and they raise 800 levies in 2 stacks, ok this is no problem. Suddenly I get the message of 'rabbles' joining their cause and they get 2 stacks of 5k, damn better buy some mercs (oh crap the Almoravids have hired them), then the Almoravids DoW with 16k+ and call in everyone and his dog. I lose the lot, game over.
I don't think an arbitary amount of levies should be given, it needs to be scaled in a reasonable way to what the faction can already raise, definitely no more than double, if that. In my example above it would appear reasonable for the faction to get 1600 on top of their 800, these are provinces with one or two slots in them and hardly any upgrades.
It works okay at keeping the HRE, France etc under control, but for smaller kingdoms it just kills them off.