These fish as a group are native to estuarine environments where the waters are likely to be hard and alkaline at least and may even be brackish. Calcium and Magnesium salts of carbonate, silicate, phosphate and borate are actually more important to the biology of brackish water fish. On the other hand if your water is soft and acid the addition of sodium salt will do little good. You need increased buffering capacity as much as the addition of minerals to the water and NaCl alone does absolutely nothing in this regard. This will adequately buffer your water and more closely match the native waters of most live bearers in question.
Salt does have other limited uses.
The resulting water will be 'softer' by the laundry definition but not by fish standards since the water now contains more total ions than it did before the softener.