All it takes is for one feeder to bring in a parasite or disease.
Many years ago I had peacock bass that would only eat food they could chase, and I fed rosy reds.
They were fine over a year, then one day all the bass were dead almost over night.
Other fish in all other tanks that ate pellets, and didn't come in contact with rosys were fine.
I do have livebearers that breed in planted sumps, and they or their fry occasionally wash into a tank, but those I trust. I've seen rosys and goldfish with sores, or glancing of objects. The profit level is so low on them, the conditions they are raised in are not always the best.