Starfish

Have a 90 gal established for allmost 2 years now and was looking into some starfish.
1) First off mixing species is ok with stars?
2) second serpent and brittle starfish. Which ones are reef safe, peaceful and don't get huge. (if I must get a huge peaceful one then I could get one small.)
3) I'd prefer an active brittle or sea serpent, but If active=aggressive Then I'll give up that idea.

Thanks for all your advice.:D