Good luck. I have sort of the same dilemma as you do, aquaholic. I have 2 angels that are about 3.5-4 inches now. One is a complete "alpha"....the whole tank is his. The other is the last of 3 other original angels, all of which (including the alpha) were introduced at the same time when they were little. The alpha killed the other 2 angels a long time ago and does nothing but pick on the one remaining one, even though they are the same size.
I can't put another angel in the tank to spread out the aggresion because it will be killed in no time flat. I tried when the alpha was only 2 inches or so and he killed the new addtion in under 3 weeks. Bare in mind, I've moved plants and rocks around several times in my tank and it doesn't seem to matter. The second the alpha sees the other angel, he goes into combat mode.
The alpha's agression has gotten so bad that the less dominant angel has had to be removed from the tank and put into a hospital tank so that he can recover. He was so stressed that all he would do is hide until I put food in the tank. Then he could come out and eat while the other angel was distracted. He wound up developing some sort of fin disease that ate away at the ends of fins, probably a combination of this stress and the constant contact with the gravel and plants that he was forced to hide in. He'd always come out with some flotsam from the plants/gravel stuck on his fins. He's recovering very well in the hospital tank. I'll probably have to bring him to the LFS and trade him in for some other kind of fish. Sucks but I don't feel like pulling another dead angel out of my tank. I think, as far as angels go, you have to either start off with a large group or only one.....that's it.
Good luck. Let me know if you find a system that lets you introduce a new angel.