Hello Pat.I only have snails in my nature pond.I do think the tiny ones get eaten if by chance i put a plant or weed from there into the fish pond.Fish will eat most things!! funny you mentioning the swan mussels.Anthony asked me about them and I said most definitely to avoid them.I believe they are only of any real value in huge lakes with mud bottoms,as the mussels bury themselves in the mud and filter their food that way.Sad to see them wither and die in normal garden ponds.
Anyway.........i would not bother with snails,they are of no real benefit unless you have a lot of algae.The waste they produce can be a drawback,and if not enough algae they will only attack your plants.Snails also will eat frogspawn.