My first cockatiel and conure were 'cage buddies'. Casey, the tiel, I had had for a month. She was given to me in November, where-as Noel came to me right before Christmas. Casey was only 6 months old at the time where-as Noel was "older"... Older, as in 9 years old? Teens? Twenties? I don't know... but adult.
Casey was fascinated by Noel and would not leave Noel alone. At first, Noel wanted nothing to do with Casey, but since Casey wouldn't take "No!" as an answer, she warmed her way in next to Noel. It's not something I had ever planned on doing, it just happened.
They were "together" for 8 years, before Noel had passed on due to failing health. Five years after getting Noel, I did adopt another older conure - Charlie. Charlie is a different species of conure than Noel but has similar colorations. He's also larger. He was 12 when I got him and in his foster home he was, more or less, "ruling the roost", in a matter of speaking. He was cage buddies with an amazon and the others got out of his way when he came by. Also due to his foster home - he's missing half of his lower beak. He decided to go pick a fight with the blind ruby macaw and got the short end of the stick. Charlie now requires beak trims for the rest of his life. He'll be 24 next year.
Casey did attempt to befriend Charlie, but after realizing that Charlie wasn't a 'friendly' bird, quickly gave up. I have no doubt that Charlie would hurt Casey, given the chance. Likewise, she has not warmed up to any other conures or non-cockatiel species since Noel's passing.
Just because I've done it doesn't mean I'd recommend it. I don't know your birds, and if something were to happen, you *have* to take responsibility for it. I lost an older bourke parakeet to Noel and it was my fault. I was cleaning the cage outside, and it had glass "seed skirts" (the design of the cage...). Well, while cleaning the cage, I had the glass stacked up and off to the side when the wind had picked up and a heavy object fell on the glass and shattered it... I didn't replace the glass immediately, but instead opted for a temporary fix. It was the worst idea ever... because I had parakeets (different species) in that cage and they figured out how to get around that "fix" and get out. Well, one of the older ones (older female bourke) did just that, and flew into Noel's area while I wasn't even home. I came back to her missing most of her upper beak. This, of course, happened on a Sunday and we have no emergency avian vets within at least 2 hours driving distance. She died the next day. One of her daughters did at least live to be ~19 years old before her own passing.
So in short, yes, it's possible, but you could also be playing Russian roulette... they could be fine together for years without issues, then one day out of the blue, there's a problem.