hi GravityCure
Welcome to Aquaria Central, home of the most old salts on the internet
The literature always states that platies, mollies and guppies do well with salt added to the water, usually in the 1 tablespoon to five gallon range. Although that does not take into account what your current water parameters are. I've always added a bit of salt (but I don't measure it, just a small handful) every couple of water changes to my Goldfish tanks, and they seem to do fine. I am currently maintaining a tank for a friend that is guppies and mollies -- the mollies are doing great and the guppies not so well. I added salt the first three water changes and to the original tank, but the last 3 changes I've added none. I'm going to go back to adding salt...
Basically, in small amounts, I don't think salt will hurt. It is VERY important though, that if you do add salt, you do regular water changes. Y'see, the water in your tank will evaporate, but the salt will remain. So, if you keep adding salt in your water, but because of evaporation, you add less water into your tank than you siphon out with your water changes, you will be building a greater salt density, and this is bad. So, you have to be a little more rigorous in your water changes if you go this route.
Sorry for the wishy-washy answer, but salt is a wishy-washy kind of subject.
Val