I honestly kept going to the male/female rather than female/female thing when I was looking for a Mika companion last week, but blew that whole idea out of the water and ended up coming home with this little shepherd mix, she's a bit smaller than Mika (40 lbs but VERY thin to Mika's 55 lbs). Mika NEEDS and LOVES being the alpha dog, basically their play is mostly Mika over Chloe's neck (alpha) and Chloe is the PERFECT submissive dog. I haven't even had one moment of angst with the two of them, they even share the food bowl. The cats are even fine with her.

Mika has a very strong play/attention drive, and since I've had Chloe she take it out on her more than us. They all sleep better, so do we! Chloe is only 2 years old (to Mika's 4.5) but Chloe is the calmer dog when it comes down to it. She's the perfect foil for Mika.

Good luck. I honestly didnt' think I'd come home with a dog like Chloe, but it's great! I don't know how you choose, but based on this experience, I don't hold much stock in the male/female being the only way to go.