Agreed with the others. Despite there smaller size, they can be a very aggresive Tang, and easily kill tankmates larger than them. With a breeding pair, this seems to go double.
I am one who have added some target fish for the very reason TaffyFish said. To distract the adults. When actualy breeding, the male really likes to do a number on the female, but with the target fish, his aggresion is more spread out. So if everything is working for you, I would leave well enough alone.