Green Chromis would be a good choice. They are peaceful, hardy and stay fairly small 2-3". They seem to do well in small schools. You tank is a little small for this, but you could probably start with 3-5 and then still add your clowns later. They are best kept in odd #s. I have 5 in my 75gal with a pair of maroon clowns a Domino Damsel(slightly aggressive) and a fire fish.
Like they said check out your LFS
or search the web for what you like.
Here is a good web site to check out pictures and brief descriptions of what is out there: