I think you got lucky here, older chicks usually don't 'get it' to go under mama.I put 5 chicks that hatched the week before in with her and the little babies taught them that she was mama and she accepted them.
...and with chicks that are only a day old.Yeah, if you want to try grafting chicks onto an existing brood, it is best done in the first 24-36 hours before the hen abandons the eggs.
