Personally I have found aquarium salt to be way over rated and even more over used!
It should only be used when a problem arises.
As for snails and Aquarium salt, They can tolerate ALOT, as long as its pre dissolved before being added,
Or make sure the snails is on the oppasite side of the tank when you add it, "They will avoid it until it totally dissolves as it hurts then to walk on it, Poor things, can almost hear them cry if forced too. But once dissolved no problems,
I have zillions of snails, especially apples,