If you use Avian Biotech (
www.avianbiotech.com), it's about $20/bird for DNA sexing. Email them for a kit and they will send you a card. You just clip a toenail until it bleeds and put a spot of blood on the card. Be sure to have the powder that stops the bleeding near by.
GCC are so adaptable that I wouldn't worry about housing them together. I actually worry about birds that are by themselves as they are very social. I have breeders that beg to be petted while their mates are on eggs (unheard of in many other species). If you do have opposite sexes, you may be OK as long as you don't give them anything to lay eggs in. An open sleep box doesn't work, I know from experience, LOL. I don't think they would lay in a sleep hut.
I can tell my pairs are going to nest when the male starts getting nippy and the hen gets shy. All summer my GCC are allowed to fly free and then don't lay eggs then. I've had as many as 12 birds sleeping in the same nest box.
So, I'd guess as long as you give them plenty of out of the cage time and no place to lay, you should be fine.
Jean
Specializing in pyrrhura conures: all green cheek mutations including pied, roseifrons and fiery shoulders.