It's alright to have one, like someone said, they may nip at their little antenne or try to eat their eyes, but snails are really good at protecting themselves. First off, they have that flap thing and can shut really hard. Secondly, they grow back their eyes or antenne or whatever, and I've seen my snails get fast at pulling their eyes back in when my catfish gets near, because one time he tried to swollow it whole. and when my minnow was still alive, it'd try to eat its eyes and i noticed the snail got really fast after it's stuff grew back.
so, while its best to have it with a mellow creature, it should do fine over time.