According to my books, mollies and swordtails, along with platies are not tight schoolers, even thought they are classified as peaceful and social.
This means thay can be kept in fairly large groups, provided you have enough room in your tank.
However, male swordtail might end up confronting each other. This can be avoided keepin mixed groups (males + females).
Also, remember mollies are a lot more supceptible to ich then other fish when kept in a totally freshwater aquarium.