Either sex is ok and it only comes into play if you begin adding more. If you may ever consider 3+, I would go same sex. all boys neutered.
boy and girl is definitely fine.
boy and many girls can be fine. but usually a single female will become alpha and there can be female fights even with a neutered male.
And many people have successful colonies with various multiples, but my experiences suggests that after you toss a female in with boys, the boys eventually begin fighting.
You never really know what you are going to get when you begin merging critters.
Boys are more dramatic than girls and will just be more loud, crabby but less actual fighting. Girls are often vicious and can fight terribly without any warning. You can find girls balled up and clamped onto each other and they arent even making a sound...