In a marine tank mollies should be fine (obviously) as far as salt and hardness go. The "shimmies" is 99% of the time a problem with mollies kept in freshwater aquaria. There's some debate about whether it's the marine salt itself the mollies need or the hardness/alkalinity that goes with it, but either way, keeping mollies in brackish water usually works well. What's the pH?
My understanding (which is limited) is that the shimmies is a symptom of neurological damage. It's possible the damage was done before you got the molly, I suppose. Provided the molly is feeding and not otherwise damaged, it should get better itself. I'd definitely increase the temperature, too. Mollies like a lot of heat, at least 25 C, and probably nearer 28-30 C.
If all you're keeping is mosquitofish and mollies, I'd be tempted to drop the salinity a bit. You're probably spending more on salt than you need to. An SG of 1.005 is plenty even for brackish water livebearers. High SG means less oyxgen, which is obviously bad, especially if you superheat the aquarium to a balmy 28 C for the benefit of the mollies.
Cheers,
Neale