I was shocked to find that I COULDN'T find Pacus in my book! I could've sworn I've seen them in there somewhere...
What I do know, however, is that the Pacu is, indeed sold at my local petshop. It is listed as semi-aggressive, but I hear that it can get pretty huge.
Sorry if I wasn't much help...
P.S. Another predatory fish that looks pretty cool would have to be the Pike Cichlids; they only get from around 6 to 8 inches in length, as well, so you would be able to keep them in a smaller aquarium.