Right it won't work on goldfish and rosies since they do not tolerate salt at all, and technically speaking, most livebearers are brackish water fish that can tolerate either salt or fresh water. Mollies actually do tend to develop parasites in freshwater and tend not to thrive at all in the long term, that's why I never recommend mollies as a community fish (also they're jerks to other fish, least the males are).

Which frog are you breeding feeder fish for though exactly? No frog is really obligate piscivore, aquatic frogs actually eat few fish in the wild. In fact most try to avoid waters that contain any fish so they can more easily breed without being preyed upon. Also fish compete with them for food, so if an aquatic frog can avoid it, it's going to try to exist in a fish-free environment.

That being said, the risks of parasites is pretty high with feeder fish. It could be done, but I think it would be far cheaper and less work to breed insects and worms for frogs and since aquatic frogs do best on diets of earthworms, insect larvae and adult insects it's the best way to go IMO.