Just wait until the younger ones are around 4 months old or about the same size is what I would recommend. I didn't get a chance to read what others have written here, so I apologize if this was already given to you:
https://www.backyardchickens.com/web/viewblog.php?id=2593-adding-to-your-flock
scroll down for adding to the flock
https://www.backyardchickens.com/web/viewblog.php?id=2593-adding-to-your-flock
scroll down for adding to the flock