Here is my advice. First do not breed unless you have proven and pedigreed chinchillas and breed for quality. It is not something that should ever be done casually. You may consider joining MCBA or ECBC as well. I strongly encourage breeders to go to chinchilla shows if possible and actively show their chins so they can be evaluated. That is my opinion on that.
Second, I recommend not pairing with a different chinchilla at this point. There's nothing wrong with having separate cages in the same room. 15 years is very old for a chinchilla. He may not be able to defend himself against an attack from a younger one if an attack occurs.
The most accepted procedure for pairing two chinchillas is to have them together in the same room for two weeks. Then introduce them in a dust bath or a place where you can see how they'll interact. Then put them in the same cage once you're satisfied they're not going to pounce on each other and start tearing each other up.
The only thing that really matters with combining chins is the personality/compatibility with the others unless you have a much larger than normal cage where the chins can basically spread out and have their own territories. But no matter what you do as long as there are multiple animals in a cage there will always be a risk of fighting and death from complications resulting from wounds. Always, even if they all seem to compatible at first, and years can go by before something touches them off. So it is your judgment call.
I would recommend staggering the introductions by a day or so, ensuring each new introduction looks legit before making more chaaos. You may also want to weigh each chin ever other day for a while to make sure they are all eating and having access to food and water, and certainly have two sets of water and food dispensers if you're going to have more than two chins in your cage.