If you keep on top of your water changes a pair of clowns and maybe something small would work.
I would pick from the following:
Firefish (Purple or Red)
Yellow Watchman Goby
High Fin Red Banded Goby
Clown Goby
Tangaroa Goby
Greenbanded Goby
Longspine Cardinalfish
Pajama Cardinalfish
Orange Stripped Cardinalfish
www.liveaquaria.com has info on all of these fish, that's a great website for pictures and info.