When I lived at Hickam AFB in Hawaii, we used to catch them by the hundreds for feeders in a drainage ditch across the road from the beach. At high tide the water from the ocean would come into the ditch and fill it up, at low tide it would be almost empty. Some of them actually had some decent color. Almost all of the females were gravid. If I were going to breed mollies, I'd try to recreate that environment. Brackish to salt water, much vegetation (not sure what to use for that; it was all weeds) and high temps (this was in full sun and the water usually felt very warm).
