Have you considered the different feed requirements as well? My babies are never introduced until they are ready or close to ready for the same feed. Hens need a layer ration, where babies need high protein grower. The layer ration could cause problems with the chicks development. Just something to consider as well.
I also fear the size difference and have seen baby chicks get killed by older chickens, so in my opinion it is not a good idea to just put them in at that age. Ours are almost the same size as the adults when they are introduced, so they can at least stand their ground a bit without being stomped to death.
Jody