Well the general wisdom among those with real knowlege seems to be that needing salt for mallies is somewhat of a myth.
Somewhat in that mollies from the wild (which you almost certainly do not have) may well come from waters which very from Fresh to Ocean Salt, averaging at Brackish -- the ones we mostly buy never saw any of that environment and have no need of salt. There are lots of articles on this (and the opposite point of view).
I have kept mollies on and off for years -- my current tanks since I restarted FW last year have 3 mollies, no salt and they are quite healthy and appear quite happy. I would not concern myself with salt at all.
It may be that mollies would prefer a higher GH/KH, but I have never worried about that either.
My tank with molllies has -- 3 bala sharks, 4 h&t light tetras, 4 guppies, 1 angel, 4 whiptail cats, 3 swordtails and 3 mollies. All have been very happy for 4 months now, no illnesses, no deaths -- nobody even looks a bit unhappy.
