I would stick with idea of, if they wouldn't eat it in the wild, you probably shouldn't feed it to them. I know canned Salmon and canned Sardines are on the "questionable" list of food items, so I can't imagine shrimp would be better. It's not something they would naturally come in contact with and plus, shrimp are rather filthy little creatures. They're bottom feeders, and if they were bred in captivity for food production, I'm sure they're laced with hormones and antibiotics to promote growth and ward off infections.
Not sure about the snails.