they will eat ANYTHING. I had mine on flakes and pellets both. Also, for fun, thaw some frozen bloodwroms in there and watch the frenzy begin! A super fun fish man.
In a 40 gallon breeder I would do 15. They do better in large numbers and with good filtration it would be fine. The more there are the less aggressive they become with each other. I have seen them kept with Red Belly Piranhas but I don't think anything else would stand a chance. Feeding frozen food is healthy every other day and a whole shrimp will start a nice feeding frenzy.