At five or six months old, they aren't babies. They are full grown as far as your adult chickens look at them.
It is unfortunate that you had your adult flock eat your previous chicks. I gather that's the reason why you waited until now to try to introduce them. However, it's much harder to integrate fully grown chickens into the flock than baby chicks because baby chicks pose no challenge to the adults, whereas these large ones will.
With any new additions, you want to keep them segregated until they all become acquainted. They need to be able to see each other but still remain safe. If you had done that with your previous baby chicks, they wouldn't have been killed. Baby chicks can be offered access to the adult portion of the coop and run at around three weeks as long as you provide a safe pen for them to run back into for safety with chick size openings that the larger chickens can't fit.
After a period of time, when it appears the adults are mostly ignoring the youngsters, then let them have access to the adult areas, but you need to provide plenty of perches and escapes for them if the pecking order gets too wild. Several food and water stations also help.