Two dogs

Can two girls live together in harmony or is it best to have a girl and a boy?

Yes two girls can live in harmony, but you will always get a more dominant one who will project an alpha female status.

IN my house we have 1 female Rottie, 1 Male Aussie, 1 female chihuahua, 1 male chihuahua. And it is not harmony ALL the time but for the most of the time is great. You have to let them know what is acceptable and was is not.

YEA I HAVE 2 MALE DOGS ONE IS GOLDEN RETREVER AND ONE IS ROTT WHILER SO YEA

Yes , two girls, especially if spayed, can live in relative harmony, if the human leader is a clear alpha. However, if you're not dominant all the time, they are likely to squable over alpha position, and could get in some real bi8ch fights. Bi8ch fights are usually much worse than dog fights, more serious, more damage.
If the dogs you're talking about are large, or just have powerful bites, or are ratters, or fighting breeds, or protection breeds, or too different in size and power , you can have FATAL fights. Lady down the street had two German Shepherd females, nearly kill each other, both spent a week on IVs and had over 400 stitches each. Females are much more serious, and more deadly fighters than males. They can shred each other. I've seen small girl dogs try to kill each other, and had time to break it up, without much damage, just because they were less powerful. Two dogs of the same sex are likely to test each other all the time to try to prove who is alpha. The best doggie buddies, are always a neutered male, and spayed female. Then there is no hormonal rages, no battles or bickering for alpha dominance. Peace.

we have a female dog and we fostered a dog that was a female and she HATED that puppy for days but she never snapped at her and finally she sorta just gave in a actually they grew a bond and we almost adopted that day, but obviously my dog was dominant

yes can do. nuter