In Kymanto Hall, it shows how you can build up reputation/faction to get 3 more slaves in the 2nd tier to be bought. But is there a 3rd tier after that one that you just can't see also that might open after? And does this mean every other faction also if you join and build rep could buy up special characters from also?
There is currently not a third tier to be bought. Once your standing is high enough they become your vassal, adding a portion of their power value to yours. That is how all factions operate, although most cannot be made your vassal at the time. I'm sure that will be added in future updates. As for purchasing slaves from other factions, I don't believe there are any outside of Kymanto and the two slave markets.