I sell 6 to 8 week old chicks every year. They are fully feathered at this age and don't need heat unless your nighttime temps are really cold.
I gradually reduce the heat for a week or so before selling them by putting them in a coop with a small heat lamp. It's cruel to take them from a warm brooder into an uninsulated and unheated coop when its freezing out. I usually figure from 6 weeks old to 7 or 8 weeks old is the hardening off period. After that they should be able to handle any temperature that an adult chicken could take. If your chicks are coming from a warm brooder it would be best to have a small source of heat for a week or two. Even a 60 watt bulb would make their transition easier.
Find out what your chicks have been eating and try to get the same feed for them. They should stay on chick starter or pullet developer until around 4 months old.
The chicks should not be put in with adults unless they have a place to escape to. Another person on here posted about their rooster killing all 9 of her 5 week old chicks. This is rare but it can happen. And hens can be pretty rough on little ones. If there is a way to keep them in the same coop but separated, they could get used to each other gradually. Is your coop large enough to fence off a small area for the chicks? I use a separate coop for the young ones I keep until they are at least 3 months old.
Hope all this info helps!
