I've never had any cannibalism in my snails, toward other species, fish, or other canas. I'm not sure about your shrimp pellets, Lupin. I feed only fresh veggies with no pellets of any kind, except snail cookies made specifically for snails. Perhaps it's the protein that sets them off? Mine get only kale, collard greens, carrot greens, lettuce...
They never tried to attack my goldfish when they were in together, either. I'm wondering if it may be genetic - some populations are cannibalistic, others not?