As far as I know- companions are Betta fish dependang, meaning the temperament of your Betta decides the outcome of happy companions.
As long as they get along a Molly is a good tankmate- however they do get rather large so I wouldn't add anything else to the 10 gallon. And I'd keep an eye on them both for signs of worse aggression
~Betta Boys ~
Elmo, Felix, Darwin, Kaiju, Rameses, Zeus, Romeo, Bronco, Tyson, Sol-leks, Siren