^I'm not sure what you mean by dont get too big, but I'd have to say that tangs and coral cats dont exactly fit that guidline. plus, besides maxing out at a few feet in length, coral cats dont show schooling behavior as adults.
there are several smaller dartfish/goby species who show shoaling/schooling behavior; Bryaninops natans (though nearly impossible to get a hold of and very, very tiny) hang out in dense groups around SPS colonies and their behavior mimics a school. blue eye dartfish (Tryssogobius colini) hang together in fairy tight groups. Parioglossus raoi as well show some shoaling behavior.
I have a feeling that cardinals will be your best bet, probably easier to get a hold of than either the B. natans or blue eye darts. red spots or blue eye seem to be the better choices among them.