Yes you should buy two or three more new female Bettas, if there's only two of them alone, either one of them will die or get badly beaten up. . .
I recommend you just adding new female Bettas, if you're worried about algae just wipe it off the screen, algae is usually a good sign in a healthy tank, but excessive amount means something is wrong with your water chemistry.
If the algae problem continues maybe get a snail, I recommend Nerite due to them being unable to breed in freshwater. Although they don't eat all types of algae they should be able to get rid of some.
Otos are in your tank are not recommended because your tank is too small, I wouldn't recommend Plecos either because they get too big, maybe a bristlenose one but then again I have limited knowledge on plecos.
Hope that helped you.