Hiya Six!
Snails are great to have around - i absolutely love them!!
And i think it honestly depends on the snails and the fish when it comes to them getting along. I have two, one about the size of a golfball (he's grown so fast since i've gotten him!) and the other about the size you mentioned. My fish is fine with them, she sucks on their shells every once in a while (giving them kisses!

) but she is fine with them, and i have a moor which i was told can be aggressive.
As for the salt, ya snails hate salt! If you add large amounts of salt into the tank, they will crawl to the top to get out of the water. usually small amounts are ok, but if you adding salt, just get a small container, put some tank water and veggies in it, and keep your snails in there for a few hours and let the salt do its work. They'll be happy eating and by the time you put them back in the tank, they don't notice B)