Depends which piranhas your talking about. I am asumming you are talking about red belies. I dont think you can sex piranhas how ever they have been known to breed in captivity. Wait for sticky rice i think his piranhas are breeding as we speak
You can not tell sex of A red belly Piranha untill it lays eggs .
They will breed in less than a year and usuly at 6 inches+ they are capable .
They are not to hard nor easy to get a breeding pair , I have heard of people
tricking there Piranha to breed by cooling the tank for a while (to simulate rainy
season) Then spiking temp back up (This is when Reds breed)
I never had a breeding pair but i would think the more Piranha the better the chance , In a 300gal id go 13 to 14 adult Reds some ground weeds and ground
wood for open/free swimming space . Post pic when you get um