Congrats on getting a second kitten!
Speaking from my personal experience only (as I don't really know what vets/other professionals say on this) I have to agree that the sex of the kitten doesn't really matter. It's all about personalities.
If youre adopting from a shelter, most shelters will let you bring in your current kitten and let them play with ones you are thinking of adopting to see how they get along - although I would make sure your kitten is fully vaccinated before you bring her to a shelter as even the best shelter will have diseases going around due to cramped conditions.
I have a good mix living in my house, all adopted at different ages, all spayed/neutered at different ages, and they all get along, but their playmates are ones that match their individual personalities...
Just as an example:
We got Gemini first and she was an only cat for a while. She ignores all the other cats and is a lap cat.
Apache was second, and she was a loner until a few years later when Mayhem (male) came along. After that, you would have thought the two of them were married. Never apart. They even died within a month of each other. It was almost as if Apache couldn't live without Mayhem.
Azrael and Jack, both boys adopted 6 months apart, love the snot out of each other. They are best buds and forever playmates.
Johnny, also male, was the last to come into that group and he hated everyone. Spent his days hissing and swatting at every cat he came in contact with. Now, he's gotten over that and has joined Jack & Az's playgroup.
So see, it didn't really have anything to do with their sex, just who was compatible with their individual personalities.
As for spaying, I've always found it was best to do it around 5 months (give or take a few weeks) so they'll be spayed just before they go into their first heat. However, I've had cats spayed/neutured anywhere from 10 weeks to two years and as long as your vet knows what theyre doing, it should be ok.