The two are different species, so there's very few diseases that'll actually transfer from snail-to-fish or fish-to-snail. Be careful though, if they die, the ammonia can shoot up. You know a dead snail is dead when you've picked him up and gave him a sniff. If it's a REALLY bad smell, he's dead.
I've never quarantined any snails for more than a few hours to acclimate them to my water/temps. After being shipped I do keep them in a separate tank for a few hours just to make sure they are all moving around and alive. Don't want to drop a dead snail in the tank with the beloved fishies. Giving them an algae wafer is often enough incentive to get them moving after coming out of the shipping bag.