mollies these days dont necessarily need salt. in the wild, they are a bw fish, true, but most mollies today are not wild caught, they are tank raised and usually raised as fw fish. check with the store you buy them from to be sure (think in the uk many places stillkeep them bw, but in the us they tend to be more fw). salt won't hurt them by any means, and will help if they have illnesses or what not, but don't need it to thrive. since you have them with fish that can't tolerater salt, don't worry about it. i keep mine exclusively fw without problems.
others WILL debate this with me to be sure, but this is based on my personal experience with mollies.........................