they may be ok together, but if you are looking for stocking advice, in a reef, i would have under 1 inch per 5 gallons, and the yellow tang gets 8 inches by itself and really should be in a 75 or larger aquarium, and the 2 clowns get up to 6 inches each, so there is 19 inces in a 55 gallon tank(whidh is actually 1 inch per 3 gallons, but....i tmay work), which imo would be pushing things a little bit, but it could be done if everythign favors your tank, and that would be the only things that you really could keep
i f you get a pair of maroon clowns, get a mated pair too, they are a bit more money, but having a mated pair of them could really hep to avoid any problems wiht them and aggression
HTH and good luck
