Characters will act as if they would in continuity, so teamwork ought to be a consideration. Personally though, I agree about Mystique, she can be a teamplayer if the circumstances are correct.
I would rank Starfire as a high EM, but I'll admit, I don't know the character all that well. Someone else could speak to the issue better than me.
the late great Donald Blake
She has flight, is Space-faring, energy blasts, and has decent strength and durability. I'm not sure she would last long against Iron Man, however.