Hello,
I had a Clown Tang many years ago, it was slightly agressive when small and proceeded to get meaner as it grew. The orginial fish with him were not bothered but when it was larger I was unable to add new fish or he would pick on them until they huddled in a corner. I bought it at about a 2.5-3" size, it grew to about 6.5" when I had to take it back to a LFS. The store owner maintained tanks in local resturants and he put this tang into a large tank with a trigger that was twice the tangs size. The tang dared to become head fish of that tank and it lost but it lived in that tank for quite some time.
Despite all that they can be very difficult to get established and eat. I was lucky or so I thought at the time as I never had any trouble getting it to eat the frozen or flake foods.
I had a coral banded shrimp and hermit crabs in that tank and he never bothered them in the least. I didn't have a reef tank so that part I can't answer.
IMHO unless you want an agressive setup or possibley the only fish in the tank I would leave it in the store.