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I know that salt is necessary to balance their electrolytes but not unless you notice symptoms. Plants and fish with extremely small scales, like corys, do not tolerate it well. Remember that there is no such thing as a scaleless fish, they are just super small.
It isn't strictly necessary. Many of them do benefit with an addition of salt to the water. It depends on whether they were raised in brackish or freshwater. Freshwater raised can slowly be acclimated to it if need be.
They can be brackish fish, but mollies are mostly sold in fresh water. It would kill them if you brought them home to your brackish tank and they came from fresh water. I think they do better in fresh water anyway.
It isn't strictly necessary. Many of them do benefit with an addition of salt to the water. It depends on whether they were raised in brackish or freshwater. Freshwater raised can slowly be acclimated to it if need be.