Your cat may never get used to the new one, cats are very fickle and I think it is best if they grow up together. Otherwise you may be party to an endless UFC championship of cats.
My cat was a wild cat who happen to have a broken leg when I found him. He came limping out from under my truck begging for help. I couldn't ignore an animal in need of help and he turned out to be the coolest most mellow, friendly cat I have ever had after I nursed him back to health.
Unfortunately, he didn't come back one night after he was out. Either someone else took him to their kids because he was sooo friendly (the story I like to believe) or the coyotes around here got him. Either way, I am now now a proficient coyote hunter.
I haven't been able to get a new pet yet, still miss Smokey, and not sure I could find another pet like him. Yes call me a wuss, I have a huge heart when it comes to animals, except for those damn, bloodthirsty coyotes