Caring for and Injured Baby Finch?

I have two finches, a male and a female. After a lot of eggs they finally got one baby going on, but he was injured because of our neighbor's cat. Then they kept attacking him, so I took him to the vet. He's doing good, but his paw is black which means it's dead. I am waiting for the vet to reopen to take him back. Even if the baby finch is doing good, every time I put him back in the cage with his parents they keep attacking him hard. So I put him again in a smaller cage. My question is, when can I put him back again with his parents? Will he be OK by himself? He's, I think one month or two months old, but still wants his mother,

My second question, I have a very large cage with the finch parents and two canary birds, female and male, but they don't breed. Is it OK to keep the two canaries with the finches? I see they are doing good so I'm not really worried, but I don't see the canaries breeding they don't get close at all.