u cant tell the sex of piranhas the only way u can find out is when they are mating... which is VERY rare
and u shouldnt feed your piranha goldfish as the staple diet... goldfish has a growth inhibiting hormone that prevents your fish to grow to its potential size..
food that are good for piranhas
cichlid pellets
shrimp {raw from the lfs or boiled with no salt}
bloodworms
krill (bitesize)
occasionally beef heart (prepared and all fat cut off)
all these foods above will enhance your piranhas color and health.
i only feed my piranhas goldfish feeders as a treat (for myself)
and the feeders get quarintined for about a week before i decide it is safe for my Ps to eat them... u see feeders are very dirty can can get your Piranha sick.. i myself lost 4 piranhas due to sick feeders.
Rosy reds are minnows, not goldfish. I'd feed the piranha bloodworms, tubifex (frozen or freeze-dried, fresh can be risky), commercial foods - anything it'll take, earth worms (if you take them from your garden make sure they are pesticide-free! Just because you don't use pesticides it doesn't mean people in the locality don't...) - it'll love earth worms, occasionaly you can try beef-heart but this has been known to cause problems and prawns are probably a good idea though I've never seen a piranha eat prawns before . If you can get a 10 gallon, sponge filter and small heater and a few guppies you can breed your own feeders so you know they are healthy.
Earthworms are great. My red bellies love them. Make sure the eat them right away if you have sand otherwise they will be gone in 2 seconds and die under there. I get my worms from my lfs.