Archmage, Well that depends on powers but if you've ever watched Justice League Unlimited and saw how grounded even the most powerful Leaguers were that should be a good indication. There's no set limit really, it's just a matter of if their power levels would break the game or not. 

And yeah that'd be fine. 

Yog, Could always use a good elemental, psionic, or energy projector. Mostly getting magic types or close-up guys.