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Definitely wait until they are all at least 5 weeks old and fully feathered before they go outside. Also, wean them off the heat lamp at 4 weeks old and get them used to indoor temps. Next, take them outside during the day for an hour for the next transition. If they start to huddle or look cold bring them in.
When you take them out to your flock they will have to live under a cage for a couple of weeks so your current chickens can get used to seeing them and then be more accepting.
NEXT, when they are let out with your flock make sure they have a place to run and hide to get away from the big chickens if they try to go after them.
Hopefully someone else will post if I missed anything! Here's an article to read too: