i was gonna say if you took out the guppies to get a pretty betta. but i guess you dont wanna loose your guppies. i think youre pretty stocked anyway for a 10 gallon, so unless you took out some anyway i wouldnt add more.
Not all mollies are brackish water and brackish conditions is not a necessity for most. They can do well in pure freshwater provided water remains hard and alkaline. By the way, they hardly are beginner's fish to me. They have the tendency to show unusual signs of health issues before any other fish can.