I've been doing this for ages, when i do a water change i should add 8 tablespoons to mine but cut back on 2 so i'm adding 6 and i use API Aquarium salt (crystals) i had some hot water and dissolve and add it to my fresh water.
No, not all Poecilia species live in brackish water. I've never really categorized them, but I feel safe saying that there are at least as many species, if not more, that never see brackish water.
I've always had harder water and have never used cichlid salts, so I'm not sure of the composition of said products. I know some are advertised as specifically for Rift Valley cichlids, some are advertised specifically for CA cichlids, and some are just generally described as "cichlid" salts. Other than that, I don't have any experience so I can't say.