Mollies will work, too. You will hear some conflicting info about them, though. Some people say they must be kept in brackish (slightly salty water) and some say they are fine in totally freshwater. I personally think they do fine in freshwater as long as they are carefully acclimated to it. Guppies may or may not work, it depends on the betta. If you do guppies, be sure to get 1 male guppy and about 5 females. Or get all females. Same with platies and mollies. You always want a ratio of 1 male to several females.
Tip: If you have a huge algae problem, you might want to get 3 or 4 oto cats. They eat algae. You have to feed them algae wafers when there isn't enough algae but otherwise they're very easy maintenance. Make sure you have a really good filter if you get otos.
I'm glad this info helps you.