Any bird, even hand-raised ones, can learn to be birds again. But it doesn't just happen like that, it would take a lot of work and dedication (which is what parrot ownership is all about anyway), and knowledge.
As an example, from the Cockatoo rescue and sanctuary, who did not believe their hand raised "pet" Umbrella toos could ever be happy in an aviary:
We have several “pet” Umbrella’s that were obtained years ago prior to opening the Cockatoo Rescue and Sanctuary. We will use one of them, named Monkey as an example. Handfed from day one, Monkey is a typical female Umbrella, snuggly, loving, babyish and a bottomless pit for attention, a Velcro bird. She would sleep under a blanket while we watch TV. I never thought that she would enjoy living in a flock, she is too needy. A few years ago, we ended up placing our Cockatoo pets into the colonies while we did some house painting. The first few days the birds pouted. It was similar to little kids the first day of kindergarten. Mom don’t leave me! Then after a few days… they started having so much fun… they did not want to leave. The same thing these pets did.
Monkey, like the other pets, now prefer living in the colonies. Occasionally, we will bring them in for dinner and a movie. The birds make it clear that they would rather be out in the flock, their new family. We have observed this over and over again,… given the choice, the birds prefer their own species to humans. Human arrogance makes some people believe that birds would prefer living with us.
So... maybe I'm being naive, but any bird can, with time and the right way, learn to become a bird again. HOWEVER, I am NOT saying, "build them an aviary and throw them in together and see what happens", definitely not.
If one of them is bonded to humans and the other territorial, I would definitely have them in separate cages for months, carefully evaluating their behavior, let them out together under strict supervision after perhaps a couple of months, and then move slowly forward.
Then there are birds who just don't work well together, who just can't stand each other, just as some people are not meant to live together. That happens even in the situations like the one mentioned above, with the cockatoos in huge outdoor aviaries.
Too bad there is not a lot of info about this, since the overall view on how to keep parrots is to keep them as separate, isolated "pets" in small cages (1-2 meters wide is still tiny, even for smaller parrots), to be kept only for human enjoyment. So either you find no info about keeping parrots in pairs at all, or it's strictly about breeding situations... believe me, I had a hard time when I tried to find info about the topic before I got my cockatoo a friend.
About the conure fighting the pionus - there may be a huge difference with one of the same kind.
Earlier this year, I was looking at a friend for my Meyers, something so distant from the Poicephalus (but stills so similar), as a Caique. This Caique was one year old, and had been bought as a friend for the lady's CAG.
That of course didn't work at all, I suppose the Caique viewed the CAG as a big "cow" or something that she could have fun with, and she loved tormenting and scaring the grey.
Then, I came over with my Meyers, Saga, to see if they liked each other.
The owner of the Caique was shocked. The bird froze in her cage and wouldn't leave it for the first two hours, she was just staring at Saga. They weren't anywhere near the same species, but the same size and shape, so this Caique saw Saga as something completely different than the big grey. After a few hours, the Caique tried to preen Saga, and it might had gone well, had I decided to take her.
However, I didn't, purely because I believe in the long run, the two species are too different.
But my point is, if your conure is fighting the pionus, it doesn't mean she will fight another conure.
My macaw hates small birds with a passion, and would kill Saga if he had the chance. But he LOVES other big birds, especially other macaws.
About breeding them - even if you do get them to like each other, please don't. There are already over 2 000 000 (no, I did not write too many 0's) birds being bred every year, in the US alone. Countless are waiting for homes in rescues, and even more are being denied a place in a rescue because there are just too many. We don't need to breed any more, if you can have more birds, then rescue some, don't create more lives into an alread overpopulated and cruel world.
