I'm wondering the same thing. I have one 5 week old chick I'm wanting to move out to the big coop. I've heard that you are supposed to wait until they're at least 8 weeks old so that they're big enough to defend themselves, but I'm by no means an expert. I have a little momma hen with her 3 day old chick that's in the coop right now and they've been fine.
This is one of the most asked questions on this forum. The standard old fashioned advice is "wait until they're the same size". But that only solves one of the problems you face adding new members to the flock. It's true they would not have any size disadvantages, but that's all it would accomplish.
The ideal situation would be that the chicks are raised within the flock from day one, and you can do this, even without a broody hen hatching and raising the chicks. Recognizing the advantages of chicks being accepted into the flock early on, I now brood my chicks outdoors in the run with the MPH (Mama Heating Pad) system. They're right there with the flock, albeit in their own secure pen, growing up within sight of the rest of the flock. Later, integration is a breeze. Read about this system on this forum. "Mama Heating Pad in the Brooder.
So this is the first thing you need to do. Set up a secure pen for your baby chicks, no matter what age they are at present. Begin letting them spend their days outside in the run with the flock so the flock can become acquainted with them.
When my chicks are three weeks old, I open small chick-size portals from their pen into the rest of the run so the chicks may begin the process of learning the pecking order. Three-week olds are savvy enough that they understand that safety is in their secure pen and they quickly run back there when the going gets rough. Don't try this with less-than-three-week olds. You should give the flock at least a week of getting to know them before you let your chicks begin to mingle with them
By age five weeks, I move them into the coop with the big chickens. That's a several day process of transitioning from brooder to coop, and it can be stressful. It helps to move the chicks in with some portion of their brooder, if not the entire thing. That lessens the stress of a brand new sleeping place.
I just move the MPH system into the coop and let them sleep in it for a couple nights, then I remove it and place them on the roosting perch instead. Each night, I wait until all the rest of the flock has roosted to eliminate interference and confusion. Also, it's best to wait until the chicks are making their "bedtime noises", light trilling sort of like singing toads make. They will be most willing to go into the coop at this stage.
You need to teach them to go into the coop at night in the beginning. It's not hard. It helps to rig up a small light inside so they can see where they're going. I just use a tiny flashlight. Then I get inside and call them in or use treats to lure them in. By the third night they will usually go in on their own.
As far as roosting, very young chicks do have a strong instinct to roost on a perch if roosting places are available that are easy for them to access and they don't have to compete with the adult birds. A lot of chicks are inclined to roost on the supports between the studs in the coop, if they're exposed. No harm in this since they will quickly outgrow them and will soon not fit, therefore graduating naturally to the roosting perches.
So, go ahead and start the integration tomorrow! Even very young chicks who still require heat can benefit from early exposure to the flock. Just make sure their heat needs are met and they're secure from harm.