As long as he was as big or bigger there probably would be no problem. IMO They need very good water conditions. I would think with the current fish you would have a hard time with a regal. If you need a tang in there consider a yellow or purple, they are a hardier species and can take the conditions. Please don't take me wrong, I am not saying that your water would kill most fish. Regals are expensive and delicate tang, the fish you have now are messy eaters and the water conditions can not be kept perfect. I know , My aggressive tank water is always a little off, and never perfect by any means but they are hardier fish that can take it.