This is a very loaded question, as there are tons of variables that can lead to aggression in a centrarchid tank. The number of individuals, their sex, their condition, size of tank, etc. I will go on record, though, of saying that they are aggressive. I had one kill a bass twice it's size. The poor thing just got the poo beat out of it. Then, I have two others in a different tank that are the most laid back fish you've ever seen. So, it depends.
As far as legality, check with your local game warden, or check the fishing regulations. They are generally what governs your ability to keep a native. There are states, like california, that restrict transport of natives, effectively baring native fishkeeping, so you should check. I believe a topic is stickied on this above.