Your mollies are quite particular about the quality of their water and are not as easy as platies. They do not need salt but a good hard water with a pH of at least 7.5 is a must. If you use some sea salt, like you would use for a reef tank, it will raise the pH like that and provide added mineral content to the water. For people with soft water, it is one way to give the fish what they need and leads to stories about how mollies always die so easily without salt. I must assume people spreading these stories live in soft water locations. I run my mollies with absolutely zero added salt and they thrive, but my tap water is moderately hard, over 10 degrees, and has a pH of 7.8. Platies are not only a different species but a different genus, they are Xiphophorus sp. while mollies are Poecilia sp. Common platies and common mollies are both hybrids within the Xiphophorus and Poecilia genuses (geni?) but are from unrelated groups entirely. The platy family includes swordtails so a mix with swordtails can produce cross breeding as Betta Boy said. The mollies can theoretically cross with guppies but I have kept them together and never seen such a cross.
