99% of the time they will fight their own species. You get the ocasional odd one that doesn't, but it's really not worth the risk.
As for other fish? It depends. Sometimes they don't care about other fish, sometimes they do. They won't be as aggressive to the others as they would to their own species, but if you're planning on putting one with others keep an eye on it and make sure you've got something spare to keep him in just incase things do turn nasty.