Hi CountryGal,
Yes, you will have a much faster and better bonding with the young one, but don't despair of your rescued mature Green....the fact that he will actually sit on your arm, even if for a little while, speaks volumes! I recently rescued a blue mature male, and although he has settled in nicely to the household, and seems extremely happy, we are still unable to touch him or handle him at all. At best he will take sunflower seeds from our fingers, and so long as we don't try to touch him he's happy enough. We too are soon to get a baby IRN to join our family, and our hope is that eventually when Skittles see's us handling the young one, he may lose some of his skittishness toward us.....but even if he doesn't, we will still love him and lavish him with attention.
Just be wary when you first bring your new baby home. Don't house them together for the first month or so, longer if necessary. Introduce them slowly, maybe keep their cages apart for awhile, and always supervise when they are both out of their cages at the same time, just to make sure the older one doesn't react badly and hurt the wee one. And make sure you spend fairly equal amounts of time with your mature one also, so he doesn't feel left out, and get 'jealous'. You will know soon enough if they will get along with each other or not. Not all IRNs will get along, each has it's own personality, and like human's, sometimes they just don't gel
What colour is the baby you are getting? We can't wait to bring ours home, once she has been fully weaned by the breeder