I made a separation for my 2 week old Runners and newly hatched by using a scrap piece of hardware cloth wrapped the edges in gorilla tape and that became their separation.
At least have them side by side with some kind of clear separation. That way when your youngest are around 3-4 weeks they may be ready to go in with the older ones. Plus once your new babies are over shipping and on their feet good you can let them be together as long as your there in case...