It can be difficult to introduce a younger peachick to an older peachick anyways. I don't think I would try keeping them together until the younger chick is strong enough to run & fly away from this eye pecking peachick. I have an older peachick and a younger one and I don't keep them together at all because the bigger one picks on the young one and the young one shakes in fear every time it sees the big one. instead I take them outside to run around and if the big one tries to peck the little one on the face I push him away. With each outing they are getting better at tolerating each other. Soon I should be able to keep them together. Watch out for your own eyes as well. I like putting peachicks on my shoulder but eventually they get curious and want to try pecking your eyes.