Your mollies will be fine. They'll probably die before even reaching more than three inches unless you're getting them from a store that cares about the fish they sell. Nonetheless, I have never had mine reach a little more than three inches. However, I would suggest getting a bigger tank if you want more fish as mollies like breathing room. You can get away with maybe another Mollie, a guppy, a platy, a swordtail, ghost catfish, dwarf frogs, snails. Any one of those would be fine. If you upgrade your tank you can add hiding places for smaller fish like tetras or make room for a bigger fish
DONT add a betta unless you want to find your fish dead the next morning. Even females are a little territorial.
Before your buy the fish look up the parameters to make sure they're the same as the mollies
I'm not sure if catfish require the same pH level