I have 3 mollies, a male and 2 females. I have a black (male), a spotted (female), and a silver. The silver is most "dominant" yet she never nips my betta. The spotted is the most subordinate, and she nips A LOT. The male, I have caught him nipping maybe twice, he mostly does it when the betta's tail is in the way during feeding, so I minimize it by feeding them separately. The spotted one, I considered taking her back to the store, but she is my daughter's fish, and this tank was their present from santa. Before giving anyone back or getting a new tank, I'm trying more cover, so I'm ordering more plants and driftwood.
Any fry they have will nip too, starting around 2 months of age, IME. They are the worst nippers, as they semi-school and it's like a game to them to pick at his tail. I sequestered them to a small kritter keeper and plan to bring them to the fish store and donate them, but they told I had to wait til they were 3 months old first. One.more.month.
It's also very important to sex your mollies accurately. If there is indeed one female, this is a very bad set-up. You need 2 females for every male, or the males will harass her to stress/death.