I'll have plenty of quests with all of their characters so while some characters do something, the other ones will do something else so two game sessions would take place at the same time game wise so exp. isn't a problem.

And for the "disallowed" classes I explained why they weren't in the begining choice but they will be able to have characters of those class later.

And may I ask what type of game is the Dragonstar RPG and what is the old Dark Sun box set?