i think a small yellow coris wrasse is a great addition to a small aquarium, they are usually accesable small at most stores and add a splash of color to the aquarium, they are also known to eat flatworms.

other options would be
randall's goby,
clown goby, neon goby, tiger goby,
small midas blenny,(can eventually reach 6-8 inches, but mine has grown possibly .5 in in 2 years, and i feed that tank up to 5 times daily spectrum thera a) mine does great in a 20 reef i have.
small pygmy angel,

there are lots of other types of small fish that would be a great addition as well, but there a bit more difficult.