I have 2 male cats and 2 female cats, all have been spayed/neutered. The 2 males are best friends and play together all the time, while the females tolerate each other, and they tolerate the males, but they aren't close.
I think the advice about getting one male and one female applies more to dogs, who are more aware of sex and pack order. We have a male dog and a female dog (neutered and spayed, of course) who get along fabulously together. But the times we've had foster dogs, if it's a male our male dog will tend to have trouble with him, and if it's a female our female will boss her around. So I think it's more of an issue with dogs than with cats.