Well Red Bellied Piranha's are schooling fish, even though they sometimes eat each other. There pretty aggressive fish which will eat things like bloodworms, ghost shrimp, pellets, flakes, feeder guppies, goldfish, krill. They will eat pretty much anything.
IMO the best way to feed live fish to a Piranha, is feed it something it can eat in one bite. Or a few things it can eat in one bite or two. Reason for this is, your more likley to have fish decaying at the bottom, resulting in algae outbreaks and stuff like that. Keeps the tank cleaner and healthier this way.
As I said before they are schooling fish, but if you plan on getting more then one buy them all at the same time, as they seem to do better when there all introduced at the same time.
Example: My first Piranha I baught, Duke, well I decided to get him a friend about a month later, and I did. Placed him in the tank, and Duke ate him.
That's why I kind of stress the introducing at the same time theory, not saying that will happen but it could, it did to me.
There tropical fish, so temperature should be around 78-85 degress, they like lots of shade and hiding spots, they don't like the light that much. I don't even use the light in the tank I have him in.
I'd reccomend just keeping Piranha with there own kind, however you can find other suitable tank-mates for a Piranha, but survival is never guareenteed.
And yea they eat people J/K, but in my opinion the movies are corny, I got enough of them when I watched Piranha II, seeing Piranaha's swim AND fly was just a bit to much for me. Actually in my experience about Piranha and people is piranha's are way more scared then people, so I wouldn't worry about the Piranha attacking your hand or what not. This is my two cents, hope it's helpful.