When and how should I integrate them so I can stop with the separation chore twice a day?
"When" is a process, it takes time. You've already started with how you have housed them but you may have a way to go. "How" will depend some on how much room you have inside and outside and when they have access to that space. Your location is in your favor, they should be able to spend all day every day outside if they want to. If you have enough room they should want to.
I have read somewhere on here that someone put them in with their birds at night and the next day everyone came out of the coop and lived happily ever after.
Should I do that? What’s most successful?
I would not yet. Sometimes these integrations go so smoothly you wonder what all the fuss was about but sometimes it can be a disaster. How much room you have, how it is laid out, and the individual personality of your chickens all influence that. Sometimes that does work, sometimes not.
That main run is not huge for that number of chickens. I'd add clutter. Clutter means putting things in there for the chickens to hide under, behind, or above. Something to break the line of sight. And have widely separated feeding and watering stations so they can eat and drink without being bullied.
I'm sure my situation is different to yours. I have a lot of inside room, actually three different coops/shelters where they can sleep. Outside I have a run 12' x 32' plus an area about 45' x 65' inside electric netting. My laying/breeding flock is typically one rooster and 6 to 8 hens, but during the summer I may have 50 chickens in there, mostly various aged male and female chicks growing to butcher age. Some of those chicks are brooder-raised, some broody-raised.
My brooder is in the coop, chicks go straight there from the incubator or post office. So my brooder-raised chicks are raised with the flock, much like my broody-raised chicks. When those chicks are 5 weeks old I open the brooder door and let them mingle. The older ones will be outside all day, the only time one comes inside is when a hen goes to a nest to lay. Sometimes those chicks are outside within a fairly short time, sometimes they may stay in the coop for two or three days before they venture outside. When they do go outside they avoid the older birds. I may have the equivalent of three or four sub-flocks out there, each staying away from the others but peacefully coexisting. Some of them may sleep in shelters instead of the main coop.
That's how I integrate the brooder-raised chicks, just open the brooder door in the morning. The older ones are already outside out of the way. I do this on a day that I can pay attention but it's never been that much of a problem. With your smaller space it could be.
My younger ones don't move to the main roosts to sleep until they mature a lot. With the pullets that's usually about the time they start to lay. Until then I don't care where they sleep as long as it is not my nests and is predator safe. I let them work that out at their pace, not mine. They typically sleep on the coop floor or on a juvenile roost lower and horizontally separated from the main roosts and above my nests. There is a lot less stress and drama for me and them as long as I don't try to force them to sleep together. They will get there on their own.
I lock my coop at night against predators. I wait until it's dark to lock it. Usually the chicks are sleeping on the coop floor. But sometimes they don't go inside to sleep but instead sleep in a group somewhere in the run near the coop. I just toss them in on the coop floor and lock them in. My coop is dark enough that the older ones aren't going to attack them overnight. Whether they put themselves to bed in the coop or I toss them in, I make it a point to be down there at daybreak to see how it is going. If one of your older ones gets aggressive it could be bad. With mine it's never been a problem, but I have a different flock make-up than you and a different coop. My coop has hiding places and room. I have no idea what your coop looks like.
I have never used that safe haven/panic room concept where you have small openings the chicks can get through that the older ones cannot so i can't tell you what size to make them for 7 week old chicks. I've never needed it but with some coops and some flocks it can be a great idea. We have different situations and get different results. My suggestion is to try letting them out in the morning when you can observe and see how it goes. Base your actions on what you see. It could be really easy or it may be a pain. Good luck!
