You can use your 10 for a grow out tank, but I wouldn't keep them in there for long. I think I had my SINGLE RBP in a ten gallon for 2-3 weeks until my fiance bought him a nice 29 gallon. Which is fine for just him, for the rest of his life. But if I wanted to get 2-3 more, I would have to move them to the 75. You always want to try to over-filter your Piranha tank, it helps keep their water nice and clean.
If you want them to be healthy as they grow, offer them a variety of cichlid pellets AND flakes. There's days where my P won't touch pellets, and he'll eat flakes. Plus I offer grated carrots, since they float, and they're good for him. And I was feeding catfish filets, but I'm switching to Bluegill and Tilapia.