Lola covered most of it. Chinese algae eaters get huge, need something like 30+gallons, and will kill fish. They get extremely territorial and agressive as they get older. Chances are though in that size tank it will die from internal problems before it gets old enough to be aggressive as long as it is well fed. Underfed and it will eat the Betta.
Depends on the kind of mollie but in my experience they prefer groups of 3+ and can also be aggressive. Mine ate each other's fins, I can only imagine what they would do to a betta.
Basically right now you've got roughly 40 gallons of fish in a 5 gallon tank. If you get rid of the algae eater that is still 10 gallons which is what the mollies need.