Go guppies.
Grown faster, easier to spawn, get slightly fatter then rosies, but still not too big a mouthful. Make sure to keep up on water changes on the feeder tank, and all should be well.
Depending on your fish (i know some wont (needle fish etc) but most will) i'd try getting them on a staple pellet diet though. its cheaper and healthier in the long run.
Craggy