I've kept mollies off and on for years, and never salted them. At the moment, I've 2 black ones, one of which is probably 2 years old now. Her companion died, so I got another couple, one of which died, due to my ignorance of the amm/nitrite/nitrate cycle.
By contrast, my lfs guy has young mollies in some of the salt water tanks, as feeders, and they look great in there, too.