Adding another Angel
 

Was it the flame or the swallow tail that was aggressive? My personal experience is that flame angels are aggressive. I tired to add a coral beauty and he destroyed it.

It actually was the Swallowtail. At first he was ok with her, then as time went on (after the first day), he went after her constantly. I put him in the sump for almost a week and moved the rock around. But no sooner had I put him back in the display, he was on her. She wouldn't even come out to eat. I had then decided to return her the next day but unfortunately when I got home from work she had died.:(

Sounds like you have a male swallowtail since you said you added a female as a partner. Male swallowtail can be aggressive. Acclimation box is a good idea. Get the new guy to eat and be strong(in a QT preferably). A strong fish can take a lot of crap. Potters will be worse since they are shy at the beggining.
Good luck.

Is it east to distinguish male from female? Would it be possible they are from different areas... If that makes an issue.

Even clowns won't always bond. Can lead a horse to water, but you can't make it drink, as the saying goes.

All of my experiences have varied fish to fish. All of them are different, just like humans. Some will be nasty, and some will be awesome. I would personally just try a different fish.

Acclamation box sounds like a good move. You could also try removing the aggressor for a while. Add the other one to the display for a while so it can establish some territory, and then try the aggressive one in the acclimation box.