Which fish to choose

[Kabong]

Clowns are very closely related to damsel's. Of the clowns the percs are one of the most docile though. However the aggressiveness of both clown's and damsels doesn't usually show up until they reach sexual maturity.
A pseudochromis is a good choice IMO he shouldn't have a problem holding his own. The clown goby's adept at using the terrain to his advantage, so is a fairly safe choice as well.

As for starfish the only kind that will do well in a tank your size are the brittle or serpent species, The other kinda need far more grazing space to survive for any amount of time.