You can have female bettas in pairs. Some bond real strong. Yet still some females are just killers and don't like to be housed with anybody. I have 6 females right now, but when forming a group I probably went through ten before getting a group that doesn't fight. The store was used to me bringing back the aggressors if they wouldn't stop fighting within a few days. I only had one fight get so bad as to have two girls kill a third before I could get her out of the tank. Next time you try, watch them closely to see that initial fighting isn't leading to any real injury outside of tail and fin shredding.

What kind of filter do you have for your goldfish tank? Even though your bioload is huge, a strong filter and weekly water changes should keep the water parameters at a safe level and prevent clouding.