Does gender matter? I don't mean for breeding, just getting along.
Not at all, although two males or two females can't reproduce if they are close enough of a species to breed!
Does Age matter? Would it be better to get a baby or an adult bird? We got Atari as a baby.
Not really. My first conure got along with my first cockatiel. I had gotten my cockatiel when she was 5 months old. A little over a month later, I got my first conure. I don't know how old Noel, the conure, was but I do know he was *at least* 9-11 years old, probably older. (guesstimates based on history)
Five years later, I adopted my second conure, a 12 year old. Noel did not get along with my second conure, Charlie, nor did my cockatiel get along with the second conure. That said, Charlie *did* have an amazon buddy in his foster home. Charlie was in a home for 10 years (assuming an only bird?), then in a pet store for about a year, then in foster for a year. I don't recall if I ever found out the age of the amazon, but it was also an adult bird.
Noel passed away about 3ish years later, after adopting Charlie. That year, Jayde, another conure, was born. She had been sold to a family, then returned to the breeder, then fostered by a member of this forum, then I adopted her at the age of 3 years old. Jayde and Charlie did learn to get along - to a point. They could even share a cage! If I had them on me though, they couldn't be together! Which was easy, because Jayde was a shoulder bird and Charlie is more of an arm/lap bird. She has since passed on due to cancer....
I also had Finn, another conure, for a short period of time. However, he came to me with health issues, which ended up taking his life as well.
I still have Casey, my first cockatiel, and Charlie, my second conure. I've had them share one aviary, although they are now in separate cages. Charlie isn't exactly a very social bird and doesn't tend to get along well with others, but he did learn to get along with her and Sylphie, the ringneck, well enough in the aviary. They all had room to get away from one another, and Charlie is the only non-agile bird! Casey and Sylphie fly around a lot, but Charlie climbs mostly.
Can GCC get along with a Lineolated Parakeets, or Cockatiels? Or will they only get along with other conures?
Best chance is honestly another conure, preferably of the same size! But there's no guarantee that two birds will get along, regardless of species, age and sex. That said, if you want another species, get another species! All my conures were the bigger guys (cherry head, mitred, red throat, green), not any of the smaller species. I never got another conure with the absolute intention to house them together. If they got along, then great! If not, they have their own cages! And they could enjoy their company from afar, if need be!