The salt dosage I have always seen is 1tbs per 5 gallons for what that is worth.
Salt is completely unecessary in a freshwater tank until AFTER disease shows up. It doesn't prevent disease, but it does ease stress and help gill function when disease is already there. Some will argue that it helps replenish electrolytes, but water changes do the same thing without adding the chlorides (I think that's the bad part of it). The basic question you have to ask yourself is, "Would a freshwater fish ever be found in water with such a high salt concentration naturally?" The answer is no, so don't use it unless you have disease. Salt is only preferable there because it is more natural than Melafix or the other medications out there that do nothing other than kill your beneficial bacteria.
On the subject of mollies (I couldn't resist), they are naturally brackish water fish (at least for breeding as far as I know). They will do well in fresh water. Most chains keep them in fresh water. But from what I know, mollies will have better color and be more active in many cases when they are kept in brackish water. It should also be mentioned that adding aquarium salt that they sell for freshwater treatments doesn't make your water brackish. You need Instant Ocean or something like that.