it's usually best to get the opposite breed. but if you're interested in getting the same breed, make sure you introduce them in a neutral area before buying the puppy to make sure they get along.
yes i agree. i own a boxer named Honey and i brought home a bloodhound puppy yesterday that is a female. i thought it was best because neither are fixed yet. but Honey didn't know how to react to the new dog. she just started drowling and backing away from the new dog when she would want to play.
It's generally a good rule of thumb to get a dog of the opposite sex for your dog, reguardless of breed. There's always going to be a pecking order, but most problems stem from the instict to be Alpha of their sex within the pack. Male-male and female-female companions are prone to fight for this alpha position. Female-male have a better chance of co-existing because they will not be fighting for the same position in your family pack.
If they are neutered it doesn't matter at all. We have had only males and neutered them. They love each other like brothers. I don't think the sex matters at all, personality types might.