Start treatment straight away, and increase aeration as the med will reduce 02 in the water, and don't forget to remove black carbon if you use it. What other fish do you have, any scaless fish in the tank as you could add some salt if not, good luck.
For marine salt? Or maybe you mean aquarium salt...
Mollies need to be kept in salt anyway, because of their tendency for these things. I guess you could add aquarium salt (what Wilder is referring to?) for this, but their problems would most likely go away if you gave them slightly brackish water. Check out the pinned topic in Livebearers.