Many people will tell you that you can't keep Bettas with any other fish. Bettas are really funny about that. Some will not tolerate anything in their tank and some will. I don't recommend a Molly though in a 10 gallon or with a Betta. You could try shrimp, a snail, or a small schooling fish such tetras or Endler's Livebarers. A Betta might be okay with that but it really depends of the fish. Also, you have to be careful with small fish too because some can be fin nippers and eat your Bettas fins.
And no a Betta won't breed with another species of fish.