I recently added a small Angel and the most dominant one ( I have a total of 4) in the tank, which is also larger, is giving it grief. The new one has been in there for about 2 weeks but there is no fin damage, so I just let them carry on. I have Elodea and some large leave plants in there and the little Angel goes in there to hide when it feels stressed. Does not stay in there for long, so I take that as a good sign.
I don't think seperating them is really an answer as when it is introduced again, you will have the same problem. I would be inclined to let them carry on as long as there is no damage done physically.