Not all piranhas live in shoals, in fact almost all do not. Red bellies require at
least a 75 gallon tank and 20 gallons per fish. So up to 4 in a 75, but NONE in a 50 as it is too small. Red bellies get up to 18 inches in the wild, but up to 12 (12 inches rare in captivity) but still a 50 gallon tank is far too small. Red bellies do not need to be kept in shoals of at least 6, a minimum of 3 is reccomended, I keep mine in a shoal of 4 and they are fine. So this tank is a no-no for red bellies.
BUT, a sanchezi, wimple, or a compressus will be good. (though it may be a bit too far of a strecth for a compressus). PLease refer to the pinned "Piranha Keeping 101" on this section for piranha help.
Im sorry I couldnt give you a more in depth response, but Ive been quite busy with all the piranha PM's with people asking for advice about piranhas