robbhp --
The fish you have are Selenotoca multifasciata, which will get to a least 20 cm in captivity, quite possibly a bit more. In the wild they are supposed to exceed 30 cm in length, but I have yet to see a captive specimen this size. This is the species of scat that I have personal experience, looking after them for many years while a student.
This species will survive in freshwater more or less indefinitely, but it will noticeably more sensitive to poor water quality that standard freshwater fish. Usually, this manifests itself as things like finrot and fungus, which are easy to deal with, but also things like lymphocystis and pop-eye, which are essentially untreatable (though they may go away by themselves, eventually). You will find them much easier to care for at an SG of at least 1.005, and ideally 1.010 upwards. They don't need marine conditions, though they will thrive in them. They do not need a constant salinity, and in fact varying the SG with each water change is an excellent idea.
Selenotoca papuensis is a very similar "dwarf" species. Both are traded as silver scats. Selenotoca papuensis has much thicker bands and bigger spots.
Silver scats are lovely fish. Even as they mature, the silvery colour remains rich, I think like moonlight. My specimens were completely peaceful. They don't seem to exhibit the aggression sometimes observed with archers and monos.
Selenotoca spp. are believed to have venomous spines in the dorsal. Handle with care.
As for mollies, the problem with them is they aren't one species, they're a bunch of hybrids. Some specimens do fine in hard water, others get all fungusy. The only 100% safe way to keep mollies is to keep them in brackish water; trying to keep them in freshwater has something like a 50% success rate. Spend any time on the Livebearers forum, and you'll read a lot of messages about mollies with fungus or the shimmies or fin-rot.
Cheers,
Neale