18" width of a breeder tank is better than a standard 55g, IMO.
like Zander said, one would be fine in a 40g breeder tank and if you can get a 75g then you can keep 4 adult rbps pretty comfortably.
pygos are much better in a shoal so if you do decide to keep solo piranha, serrasalmus would be a better choice than pygocentrus. maculatus, sanchezi, spiloeura or brandtii can all do fine in a 40g or 50g breeder tank.