I have had third and fourth generation mollies in salt water and they got huge. The variety I used came from a clownfish hatchery in Purto Rico.
It is my experience that mollies are at risk with most reef tank fish.
I am presently setting up a macro lagoon as a mixed garden tank to highlight ornamental macros and inverts, including coral. As a source of nutrients, I will use mollies and treat them like large amphipods as the only fish. Just as some growout frag tanks are teeming with pods, they could also be teeming with mollies.
I used to think, lyre tail mollies did best. When I fist started marine aquarium, I have netted green mollies in the wild in fresh, brackish and full strength salt. Yes, I have used PerSmart mollies with varied success. In Austin, one LFS sells Black Mollies fully adjusted to salt.