Welcome to the forum Bridget.
Aart covered it pretty well. Sometimes these things go so smoothly you wonder what all the worry was about but sometimes (but rarely) chickens die. That's the nature of dealing with living animals and their behaviors, you just don't know what will happen.
One huge issue to me is how much room do you have. When you are integrating the more room you have the better. It's not a case of square feet per chicken, it's whether or not the chickens can get away from each other or avoid each other if they need to. Some of that is the quality of your room. Having room inside and outside the coop available is better than just having one room. When there is conflict between chickens the weaker usually runs away from the stronger, then avoid the stronger as much as they can. It's pretty common when I'm integrating for the weaker (younger) to be in the coop if the adults are in the run. Or the weaker to be on the roosts with the adults on the floor if they are all locked in the coop together. The weaker are avoiding the stronger.
I have a lot of room outside so it's not that unusual for them all to be outside, but in different areas. But it's also not unusual for mine to be pretty well mixed outside when they are foraging after the initial integration is over. Each brood is different and each flock has it's own dynamics. Room, room, room, most important thing in my opinion. You can increase the quality of whatever room you have by giving them things to hide behind, on, or under. Break up the line of sight but don't create traps where they can't run away if they need to.
It usually helps a lot to house the chicks or chickens side by side for a while. Some chickens will attack strangers. Not all but some. If you house them side by side for a week or more they at least accept the others right to be there. Some chickens are just brutes and bullies anyway, they may go out of their way to be brutal to other chickens no matter how much they are housed next to each other, hopefully you don't have one of these. But these brutes are fairly rare as long as they are not shoehorned into a tight space. Crowded chickens often don't behave very well.
Something else I find very beneficial is multiple feeding and watering stations. This allows the weaker to eat and drink without challenging he stronger, which they pretty much will not do. It's a way to help avoid conflict.
We can make this sound absolutely horrible. I integrate several broods of brooder-raised and broody-raised chicks every year. My broody hens raise their chicks with the flock. My brooder is built into the coop so they grow up with the flock. I have a lot of room. I hardly ever have integration issues except when the cockerels get old enough to start bothering the rooster's hens. That's not an integration issue, that's part of them growing up. So if you have cockerels in those chicks you might have some issues there.
Once chickens accept that a place is home, they generally return there each night to sleep about dark. So house them in your coop for a while and they should return there each night to sleep. Or lock them in the coop after dark until they get in the habit of sleeping in there. After dark they are usually not that hard to catch unless you have a lot of light down there. There can be some issues with this though.
Mine are most brutal toward each other as they settle down to sleep. It's a pecking order thing. The stronger get to sleep wherever they wish and can be pretty brutal enforcing that right. My younger birds generally do not sleep on the roosts with the adults until they mature enough to force their way into the pecking order. For pullets that's usually when they start to lay. With your age groups I don't know how that will play out, you may not have any of these issues. None at all. But don't be surprised if your younger birds avoid the roosts with the older ones up there. They may look for a safer place to sleep. That night be your nests, somewhere else in the coop, or even outside the coop. Some of that will depend on how big your coop is and how it is laid out, but each brood is different for me. It gets back to behaviors of different individuals being unpredictable, they are just not consistent. I'm almost constantly integrating so I put a separate roost lower than the main roosts, higher than the nests, and horizontally separated from the main roosts to give them a safe place to go that is not my nests. Most but not all broods use that juvenile roost. My first group of pullets this year slept on the coop floor until they started laying and then joined the adults on the main roosts, skipping the juvenile roost entirely. If some of yours want to sleep outside the main coop, as long as they are safe from predators that's not a problem. As they mature they will work out those issues.
A lot of us do something really similar to what you are doing all the time, often not following everything Aart or I said. There can be some pecking and rough stuff, but usually it's not a problem, they work it out. Giving them as much quality space as you can, separate feeding and watering stations, housing them next to each other, things like that improve your odds of success. Just do the best you can an d you will probably be OK.
Good luck! And once again
