Snails should be fine if they are small enough, just make sure it is smaller than the axolotls head, half the size or smaller. Soft shelled snails are going to be a lot safer as well, try to stay away from the hard shelled ones unless they are significantly smaller that the axolotls head. Because, as LSuzuki says, they can be eaten, and can cause impaction. For a beginner though, it is best to just steer clear of them entirely.