Hi
Here is my 2cents worth, if the fish are roughly the same size put them in together. In my experience the kole tang despite its reputation as one of the least aggressive tangs has a tendency to try to exert itself on its tank mates, unfortunately it lacks the equipment and the skills so it gets beat up.
I have a tank similar to yours with a resident 4" kole in december I added a 3" purple tang ,no problem with any other tangs in the tank(yellow,Atlantic blue,convict) yet the kole went after the purple with a vengence.Needless to say the purple had no problems, the kole was bringing a knife to a gun fight after 3 weeks finnaly the kole figured it out and there is peace.
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