Clownfish change to female when one in the group becomes the dominant one. All others in the same group remained male. So, to make sure you don't have 2 females fighing, the safest bet is to get one significantly smaller than the one you already have.
HOWEVER, I think you should solve the problem with the first one before you go out get another one. IF the problem is environmental, getting another percula is simply a waste for them and for you.
