In one of my dog behavior classes, an interesting analogy was brought up by the instructor. She said to think back when you were younger and in school. Males tended to push each other around a bit and get into fist fights every so often. Females rarely fought but when they did, it was bloody.

She said the worst combination is female-female. It can be uglier than male-male. I believe this to be true from what I've seen but there are also exceptions to every rule.

If there is a notable size difference in the females, then dominant fighting issures are less likely to occur because usually alpha will go to the bigger dog.

If you wanted another girl dog, I think you could definately work it out. Maybe get a submissive one. Reece and Harley get along so well because Reece is very very submissive and Harley is very very dominant.

Good Luck