In general, dogs of the opposite sex tend to get along better with less scuffles and aggression towards each other. Two females could become aggressive toward each other, even if both are spayed, unless they are young enough, fixed and grow up together just like our dogs. Pls make sure you spayed your female dogs.
Yeap, agree with you Jesse. My 2 spayed females who are about the same age will tend to fight with each other (The elder one will bully the younger one most of the time) when they are in jealousy.