Yellow tangs can be an aggressive with ANY other fish of similar type (other tangs), similar size and/or color. I have had yellow tangs be aggressive towards angles/butterflies/and other tangs. They usually ignore the smaller fishes like clowns, blennies, etc. It always depends on the one that you get - some individuals are more aggressive than others, but as a whole, they should be considered somewhat aggressive.
I doubt one would bother your clowns, but he might NOT like other fishes added later