Bettas are tricky in a community tank. I've had success with them (always using female bettas, not males), but others have had the opposite experience. Either a male or a female betta *might* work, but it depends 100% upon the personality of the individual fish. Some are a lot more tolerant of roommates than others.
What it does mean is that you should feel free to try...but you NEED to have a "backup plan" in place in case your betta decides he (or she) wants the entire 10 gallons to himself/herself. Because a pissed off betta, whether male or female, would certainly be able to kill your mollies if he/she felt like it. Also note that aggression issues are general minimized by having a tank with a lot of plants (whether real or fake), driftwood, rocks, decorations, whatever--anything that "breaks up" the sight lines.
Also note that a general rule, for all fish, is that the more crowded you make them, the more likely you fish will begin to show aggression. If you want to try the betta, then I would add one to your two mollies and add nothing else, other than perhaps another few cories (3-4 more). Most water-column fish aren't bothered at all by substrate-dwelling fish like corys, and corys are by their nature schooling fish and will be MUCH happier if you have them in a small school. For a 10 gallon tank, aim for 4-6 corys total in your school.
P.S. -- Is your tank cycled yet? If not, you don't want to add ANY more fish until the cycle is complete.
