Wow! Thank you so much for all of the great information. I will take all of it into consideration when I begin to mingle. They will stay separate from my big girls for a few weeks yet. The littles slept in the house one last night and will be moving out to the coop this evening. I'm hopeful that things go well.I've had 5-1/2 week olds go through nights in the mid 20's F. Yours have been acclimated so those temperatures should not be the problem. It is not cold.
I'm not sure what's going on. The first time a chick is in total dark, some of them will be afraid of the dark and start distress chirping. They usually get over it within 10 to 15 minutes if you can be that patient. Perhaps that chick has imprinted on you as its Mommy and doesn't want you to go. Perhaps it is something else. In any case chicks and chickens are pretty adaptable, they will get over it if you let them. But that distress chirping can be heart rendering.
What they are talking about with that hole in the fence is sometimes called the safe haven or panic room method. If you have enough size difference in the chicks versus the adults you can create a passage between the two pens that the littles can go through but the bigs can't. If the littles mingle and get attacked they can run into their pen where the bigs can't get to them.
There are lots of ways to create that passage other than just cutting a hole. Secure the door partially open, use a wire mesh with the right mesh sizes, or raise the bottom of the fencing enough for some examples. I don't use that method, never needed to, but in some circumstances it can really help. One challenge is getting the passage size right. You want the chick to be able to run through and not get caught but some adults are mostly feathers and can squeeze through a pretty small hole.
There are several different methods or techniques of integrating. How much room you have inside and out, what your facilities look like, how you manage them all, and the age of the chicks can make a difference as to which might be best for you. The more we know about what you are working with the more likely we can come up with specific suggestions that actually fit your circumstances. Without that information all we can do is be kind of generic or tell you what we do in our circumstances.
Some of my generic suggestions are to give them as much room as you can, improve the quality of what room you have by adding clutter if it is a bit tight, have well spread out feeding and watering station, and don't force them to be together but let them work that out themselves as much as you can. Clutter means things to hide under, behind, or above. Until my littles mature enough to join the adult pecking order they tend to avoid the adults during the day and also at night when they are sleeping. I don't put my littles on the adult roosts to try to force them to sleep up there but let them find their own place to sleep. As long as mine are sleeping somewhere predator safe and not in my nests I don't care where they sleep.
I have a reasonably large coop (8' x 12') with places chicks can hide from or avoid the adults, over 3,000 square feet outside, weather they can be outside every day, and my brooder is in the coop so chicks basically grow up with the flock. I have food in three different places and water in four. Usually when the chicks hit 5 weeks old I open the brooder door and walk away. My integration is that easy. The chicks avoid the adults during the day and find their own place to sleep at night. When I go down there in the morning to let them out the littles are on the roosts while the adults are on the coop floor. My roosts are high enough that they are a safe place for the littles to go to get away from the adults. Not everybody's roosts are.
You are not in this situation so my methods won't work for you. You need to find your own solution based on your circumstances. Good luck.
I appreciate your very detailed and helpful response