Oh. My. God. What is with squabbling on every bloody thread? Sounds like a bad version of bloody Twilight.
I will agree that mollies are brackish, plus they like being in groups since they are pretty social. I have owned both balloon and regular mollies, and have had them in a 10 gallon (cycled, and well cared for), then upgraded the tank to allow more room since they were so active!!! However, if you want to try it, just make sure to have a back up plan in the case the molly gets sick (they are more susceptible to mouth fungus when in fresh water), or the betta may become a victim to a moody molly
All depends on the betta and the molly of course.
Not a combination I'd go with, but we're the opinions, you're the final say