I would stay away from that combo. I know snails can hide in their shell, but I’m pretty sure snails/slugs also produce a very sticky mucus when stressed that could be dangerous for the mantis. Dangerous in that it would take a lot of energy to clean off if it could. Personally, I wouldn’t tempt it, but if you do, good luck. Maybe research snails defense mechanisms so you know what they’re capable of. Good luck!
-mino