There have been studies that fish do well in brackish conditions, even many freshwater fish (I saw this on the discovery channel at some point). But this is more likely do to the other things like buffers and minerals that marine salts contain. The feshwater salt they sell atthe pet store in simply iodine free table salt. If I were you I would stop adding it to my water changes. All of your listed fish can do fine in freshwater, even mollies.
That salt is a great thing to have, because unlike many medications it has no expirey date and you can always have it on hand. Is it hurting your fish? Probably not but that doesn't mean you should put it in there anyway.
Good luck with the snails.