I'd use blanched romaine, zucchini or cucumber as a staple to feed them. If you have one a slice 2-3 times a week. As it is I try to put in two slices every other day for my oh about 5 quarter sized munchers.
As for fry, most apple snails, at least pomacea diffusa, formerly known as brigs, shouldn't harm any fry. Pomacea cana...(well I have no idea how to spell the rest but they are simply called cana!) may cause trouble with fry.
Having said that 90% of the apple or mystery snails are diffusa. Cana is illegal to sell in many states. They also grow larger, and will destroy any and all plants.