In the UK, before they were banned by the EU, we used the term apple snails to cover all the various species of Pomacea. There were two species common in shops, brig apple snails and cana apple snails. Brigs were Pomacea diffusa (called brigs from their old name) which stayed smaller than a golf ball and did not eat plants; canas were P. canaliculata which grew bigger than a golf ball and did eat plants.
Just so that I know, what is the species that you call mystery snails, and which is the species you call apple snails? The photos of mystery snails I've seen on here look just like the brig I had years ago.
Nerite snails - I've never fed mine on anything. They eat algae and whatever they find - the fish are fed flakes, sinking pellets, sinking algae/kelp wafers (not all at the same time). I've had two of my snails for 9 years.