Mollies and Platies do get along, in fact they get along quite well. Your Mollies might be stressed for another reason then scared of your platies. Or maybe they just need their space. My mollies hide by the rocks during when the lights are on, but when the lights go off they are all over the tank. And yes, if you have no catfish in your tank, add about a tablespoon of aquarium salt per 5 gallons of water in your tank. That will make your mollies happier also.