I would get a 3-4" Sohal, I have a 9" one in a 240gal with a Sailfin and Purple with no problems since all the tangs know who is the boss. I have enough rock and places that the Sohal can't get into if the other tangs need a time-out from the Sohal. Sohals get big quick I bought mine at 4" he grew to 8" in about 7months, i only know of one other person that has a Sohal in a tank around where I live with other Tangs in the tank. You are taking a risk with a Sohal added to the mix, but you might get the aggression dispersed over all the tangs and might get away with it. I actually have such a bold Sohal that if I need to remove him he comes up to my hand to feed and is easy to catch, did it once.