you should actually try not to feed them feeders that much. they arent very good for the piranhas. try to feed them things like white fish fillet, shrimp, squid, smelt, bloodworms...the list goes on. if your going to feed feeders you should quarantine them for a couple of weeks in another clean tank and feed them well. if your completely set on feeding feeders then yes you can put a whole bunch in there and let them eat at there own rate, but there is going to be a clean up of dead fish, and an increased bio load. by the way, welcome to the site
i feed my p's mostly shrimp and they love it. i take the shells off because they don't like it and they usually leave it in the tank for me to clean up. i have some cigar minnows in the freezer. these are whole fish that are frozen and they contain the guts and all. i was going to wait to feed them this until they get a little bigger or about 6'' or so. but anyway, i feed them feeders every once in a while now, most of the time they don't eat all of the fish anyway. they will leave about half of the fish in the tank floating at the top. i have a blender and i read one of the threads that some guys were mixing different meats and vitamins up for their fish and then adding gelatin, pretty cool idea i think. i just don't know if i still want to make some of those margaritas in the same mixer anymore!