The question I always ask in these situations is "why do you really want to do this?" It can be done, but I don't usually understand why - except in certain situations where people want baby mollies as food. And many then ask "how do I get them all out."
But a balloon molly is already somewhat of a "mutant" (not always the fastest swimmer) and may be difficult, IMO, to combine with saltwater fish who may pick at it. Not only that, but if there are a few of them you are using up limited tank space with them - you can't keep as many fish in a saltwater tank as a freshwater tank to begin with.
Personally I would really think about reasons for doing it.
