Typically I would recommend that you quarantine a new rat for 2 weeks before intros. However, if you're not going to quarantine you can start intros whenever. Younger rats are usually accepted pretty easily by older ones. Grooming is not only a bonding activity but can be a sign of dominance. Even if he's young the older ones will want to show that they are the boss. And as he gets older there might be some scuffles if he decides to test his limits.