The only shoaling piranhas are members of the pygocentrus family. I have known experienced keepers to have shoaled serrasalmus geryi together, but if trying to co-had rhoms you will be wasting your time and money.
If you want an awsome piranha shoal, I would try going with some pygos (caribes, ternetzis, pirayas, and reds) Just make sure you have a really large tank and double the required filtration because they are super messy eaters. The max amount of pygos that could be kept for life in a 75 gallon tank would be 3. 4 would be pushing it.
Rhoms are really neat fish though. Once he gets settled in and knows his territory anything you drop in the tank will become food. Good luck!