As far as your nitrate levels go, have you tested your source water? If the water you are adding after a water change has nitrates, it will be hard to reduce the total nitrate level of the tank.
While salt may be beneficial to mollies, they don't really require it.
Agreed. I've breed plenty of mollies for feeders with no salt what so ever. Captive breed mollies don't require salt for breeding. I like using 1 male per five females. Ive also had males become very agressive with each other and also had males "breed" themselves to death.