I keep a pair of P. septemradiatus in a 20 long. There was definitely some aggression to start with, however now they are pretty friendly with each other. As has been mentioned by the very knowledgeable HX67, various territories and space are key. I added a land portion with an acrylic sheet, a sloped sand hill, and a few large rocks sticking out of the water. Once everyone had a rock to lord over, aggression went way down. Very important to give adequate space and not overstock. Once I have the space/cash, I'm planning on getting a 40 breeder, and possibly adding a skipper.
These guys are easily the most personable and amusing fish I've kept. At this point they both sit on my hand while I feed them worms. They seem more excited to see me when I get home than my lazy dog! I'd recommend them to anyone