My experience with a molly is that it feeds fairly near the surface and prefers to clean glass rather than attack the caulerpa I'd hoped it would go after. Sigh. I lost one of the two, and if I had to hazard a guess, I'd say females are chancier because almost all females are preggers most of the time, and that state is probably not a plus in acclimation. But so far so good with the male, who is at least doing something constructive.
I'm wondering if the green mollies might be better than the black: the greens are probably closer to the wild state.
It's my understanding that these fish actually transit from rivers to estuary to go after algae. Goldfish would not be a good bet.