Cory cats should actually stay in a group of 6 or more, and most species will do great in a ten gallon, as long as your substrate is fine enough that it won't hurt their little barbels. Guppies are iffy, some bettas will be okay with them, but some will not. My girls do fine with them, though my male guppy is missing a little nibble on his tail. If you go that route, make sure you have a ratio of 1 male per 2 females, otherwise all males or all girls. That way the females aren't too harassed. They also have a lot of babies, so you'll have to make sure that your tank doesn't get overloaded when fry start pouring out every couple of weeks.
I'd steer clear of mollies. Some people have had success with them, but from what I've heard most have not. They get pretty large, and they like to nip fins. If you do a betta sorority, you have to have at least four bettas, and a LOT of hiding spaces and plants. They are very stressful for the fish, so if you go that route you have to make sure the water conditions stay pristine to avoid illness and sever aggression.