When I was a child, all our peeps were reared by our hens and there was never a problem introducing them to the flock.
As an adult, I've added chicks I incubator hatched (and brooded...then kept them in
their own coop and run until they feathered out before introductions.)
I've decided next year to see if any of my hens go broody they will raise their own chicks.
I've had friends whose hens raised their own chicks even late in fall(Most of them had sneaked a nest somewhere) and raised their babies with no special care or having a brooder to keep warm and clean.