Kage: it depends on how much salt you add. Yes, salt will make the bait harder. It is a good idea to buy some softner. At some point in time you are going to use it. You see, you never stop with just one mold. You keep buying different molds for different baits. Eventually, you will buy a mold and find a formula that requires softner.
As for MF SS, that is my plastic of choice. I do not use any softner with it. Saint.