i think about 2 clowns would be all right... i would prefer the perculas clown, its my favorite...and also, i think that a 55 is 2 small for a yellow tang. other fish i would like to reccomend is the clown gobies- i think the citrus, green and the rusty are all okay.also the mandarin dragonet goby is peaceful and sweet looking. i know you didn't ask this, but for a reef aquarium, you might want some live rock and some coral. i know live rock & coral is expensive butcheck out some other sites... i know a site where their coral and live rock is really cheap.
good luck!