I'd go for something like a clown, blenny (or goby), and a basslet (assessor, or gramma, etc.); I wouldn't put more than 7-8 inches (adult size) in a tank that small, especially if you have corals; fish are the major polluters in captive systems, and things can grow bad quicly in small tanks. I like the above mix of fish, as they are typically very hardy, stay small, and don't produce nearly the waste that say dwarf lion, or other predator would.

Thats my opinion.
andrew