What kind of clowns? Ocellaris and Percula clowns can go with just about any other fish. Gobies, blennies, wrasses ect.. all work pretty good with them. Clowns are damselfish so they can generally hold their own against others. For best result try and introduce them at the same time, but even if you can't do that they should be fine. The only one I would not introduce after the clown, is another clown, as that can go very badly if the new clown happens to be a female. Females will fight to the death, although males will just get bullied until they know the pecking order of the group.
Clarkiis and Maroons clowns though are generally pretty aggressive and get quite large so I wouldn't keep either of these in that small of a tank.