Just to hammer on the point further: no fish needs "Aquarium Salt" on a regular basis. I do believe it can help in the treatment of ich (although table salt works just as well and is much less expensive). I had no information on the "stress relief" effects it was supposed to have but had no reason to ever use a "stress relief" product in the first place as, if my fish were stressed, the problem should be solved rather than the symptoms (stress) treated. Cure the disease, get the water parameters in line, change the lighting, change the stocking of the tank, etc. Byron's indication that the salt may actually increase stress (which makes a lot of sense considering things like natural fish habitats) just drives the point home further.
Mollies can actually survive in brackish and even full marine conditions, and I've read from multiple sources that mollies will actually grow larger and live longer in brackish water than pure freshwater, but they will also do quite well in fresh. The salt that is used for salt-tolerant livebearers (and other fish) is marine salt mix, not "aquarium salt." Completely different product, and absolutely not something you want to be putting in an aquarium containing pure freshwater species like neon tetras.
So, long story short, you can keep salt-tolerant livebearers* in freshwater tanks with freshwater fish.
*Of course this excludes any obscure species I'm not aware of that are only found in brackish tributaries or the ocean or something. I'm just talking about the common varieties here.