I went to a seminar on Sailfin mollies earlier this year. The presenter said that the issue wasn't salt per se, it's having ions in the water that the fish can use to regulate their osmotic pressure. Now, with salt in the water, the mollies can use the sodium ions (Na+), but they are also found wild in areas with a limestone substrate, leading to hard water, though 100% fresh, water (the Everglades, for instance), and in that case they use Calcium ions (Ca++). Either works. If you're going with salt, you will do better using instant ocean or the like rather than regular aquarium salts. That will give a richer mix of ions in the water, which should help. As to the amount of salt, IIRC, the presenter said he got the best grow rates with a specific gravity around 1.002. It tails off a bit after that, bottoms out around 1.015, and comes back up around full marine.
So What to put in with your mollies. Well, if you were to up your salt content to give you a SG around 1.003 or 1.005, then you could put in a school of Glassfish or Bumblebee gobies (which you'll have to feed by syringe), or, if you have a sand substrate, a violet goby. I believe (and you'll need to verify this), that pristella tetras are found in brackish waters betimes.
If you have an aragonite substrate, you can skip the salt entirely if you like - that'll harden the water plenty. In that case, there are a number of hardwater fish you could use - American Flagfish, Kribs, the more peaceful CA Cichlids (like a Firemouth
, but not a pair), Synodontis cats, Rainbows (though they might frek the mollies out), Blind Cave Tetras, Pygmy Sunfish (if you keep the temp down in the low 70's), or even some of the smaller lake Tanganyika cichlids (a couple of shellies).