I had two girls who got along wonderfully well, but I added the second when she was 7 weeks and the older was 7 years. After a couple of days of stiff legged walking around and occasional growls, my older girl decided the pup was "hers", and they were BFFs for a lifetime after that.
My girl/boy combination didn't go quite as smoothly. After my old girl died at 16, I had the idea my 9 year old might be lonely. When I heard of a two year old boy needing a home, I provided one. Well, my girl hated him on sight! But over a few days we convinced her to tolerate him. They got along O.K. after that, but never developed a "loving" relationship. When she passed on, we stayed with one dog, and I think he was happy with that arrangement. My girl had never liked him much, and not having to share the human attention suited him just fine.