Male bettas are aggressive to each other, as you know, and can't be kept together. Males have longer fins than females. Once in a while, you'll find a short-finned male, but usually, you'll be able to tell a male and female apart by the length of the males' fins.
Females can be aggressive too, but you'll be able to keep more than one in a 15 gallon tank. Do you mean in a heavily planted tank? They will like that. They will establish a "pecking order" among themselves, and they will be fine together. As with any new tank of fish, just watch them closely at first to make sure everyone is getting along!