I have 6 girls, 4 boys.

All I ever heard as a rule of thumb was that the second cat should be the opposite sex. After that, I never heard anything, it all depends on the personalities.

The one thing I have heard is try to get only one Queen, as multiple Queens will fight. I am fortunate, I only have Vita, and I didn't know about that as I added cats. I doubt it would have changed anything, kitty needs a home, I take 'em in.

A Queen has a much larger head than other cats. Vita's head is a huge circle, and almost twice the size of the other females here. And oh yeah, she is BOSS! EVERYONE bows down to her, male and female, feline AND canine. She gets a bit nippy when humans don't respect her regal highness! But most of the time she is not trying to boss me, just get some petting so that is fine as well.

As for introductions, have the easiest one at 3 days and the longest at almost 4 weeks! Just keep the water squirt bottle handy. Keep the newbie separated for a few days, behind closed doors, and swap who is in the bedroom and who is out roaming the house. Remember the newbie has to get used to the new house sights, sounds, smells, routines, in addition to the residents!