It is very rare for piranha to live with other fish for long without killing them. They even kill each other. The other problem might be that if the catfish can fit the RBPs in its mouth it will eat them.
I would not want to risk such nice fish. You might make it work, I have successfully kept fish that should have killed each other together. I think RPB's and large catfish are one of the most risky to try it with. As I said I would not want to risk it, high risk of loosing some beautiful fish.