you could get 1-maybe 4 fish in that tank depending on size and swimming requirements.
here are some ideas
clownfish
clown goby
firefish
diamond goby
jawfish (needs 3+ inch sand bed)
lawnmower blenny
pygmy angel- may have a chance at picking at some corals..
blackcap basslet or royal gramma
yellow wrasse or sixline wrasse
bangaii cardinals
like any of those?