The more I read this forum, the more I think that compatibility all depends on your fish. I personally have a betta in a 10 gallon with a platy and a dalmation molly. They get along perfectly fine. There was like 5 minutes of checking each other out, then they left each other alone. I have also seen a few members keep guppies with a betta where most would say to definitely not do that.
My opinion is that if that's the fish you want, your tank is big enough and wont be overstocked, and you think your betta will get along with them, try it. Just always keep a close eye on them especially right after introducing them. And always have a back up plan. Like an extra tank for the trouble maker or taking the mollies back to the store.