Groups are definitely more interesting. But from what I read a solo piranha will live a much less stressed life. Piranha are very competitive and will not hesitate to cannibalize a shoal mate. They must constantly be on guard in a shoal and dare not fall ill for the threat of being eaten by their peers is very real. Solo piranha does not have to watch his back or compete for food, and although numbers give them confidence a single specimen will eventually become bold.
Shoals work in the wild for defense against predators and if you don't think being in one is dangerous and competitive pay attention the next time you feed and see if they share the available food or if its an every fish for himself free for all.
In my opinion a single RBP in a 50 with some smaller dither fish could make for an excellent tank.