I would not add clowns until you are sure there's no ammonia spike from the rock. Chances are their will be. To answer your question, maybe a year or so before your tank will be a bit tight for the fish. Maybe a Foxface rabbit, an Atlantic Blue Tang or Purple Tang, and a Flame Angel? Then perhaps a sand/rock dweller like some sort of goby or Blenny. Your not keeping corals right? Just fish? Will you have inverts?