Hi,
It is best to wait until the females are about 2 years old before letting them pair up and breed. Younger birds do not make very good parents, and the females are at a higher risk of becoming egg bound. Once they are old enough, yes, put them all in a large cage and keep a close eye on them. You need as many food and water stations as birds, to prevent any from being bullied and deprived of food. They tend to swap around a lot, but eventually you may notice pairs spending their time together. That would be when you separate them, and see if they are truly bonded. They should spend their time together, grooming and even feeding each other. If you see them mate, then you can give them a nest box. And yes, one pair of birds per can for breeding.
Thank you for asking Lafeber,
Brenda