How about a yellow tang the exact same size as him? Oh, wait... that's the WORST thing you could add!

We had a very dominant yellow tang in our 100g and added a purple the same size (they have the same body shape as the yellow so those two are the most at odds with one another)! They scrapped pretty badly until we put a big piece of egg crate in the tank to separate the two for a week. When they got used to one another, it was just fine. You'd be better off choosing fish that are dis-similar in size and shape.

Of all our tangs (yellow, purple, blue hippo, and powder blue), our purple is the "low man" on the totem pole. For whatever reason, the fish he chooses to take his frustrations out on is the long-nosed butterfly. I guess he sees him as most similar in size and is threatened. No problem between him and the wrasses, gobies, clowns, anthias, hawks, etc. Just gives heck to that long-nosed butterfly fish.

Good luck. Definitely try the rearranging of the rock work when you put a new fish in, and maybe try putting the new fish in pairs so that it takes the heat off of just one.