My two older ones are male and female and I got a second female. It worked out well, because my GSD is very good tempered and the older female is about 7 years older then the GSD. I will add a fourth dog and it will be a male. I won't push adding a third female. All of mine are good tempered.
Misty- Samoyed Mix, Tannor- Golden Retriever CGC
Robyn- GSD CGC, TC, Midnite-GSD CGC,TC, Brennan-Golden Retriever CGC, Batman-Husky/Greyhound , Apollo-GSD