This will get a little long, so bear with me. Many of us integrate and don’t have problems, but some people have disasters. You are dealing with living animals so you cannot know exactly what will happen.
Since you don’t have roosters, you would only have two different kinds of aggression to worry about. First is just territorial aggression. Chickens can recognize which chickens belong in their flock. Sometimes, they will protect their territory from strange chickens. This does not happen each and every time, but it does sometimes happen. This is where housing them side by side for a while can really help out. They learn to recognize each other without fighting. Quite often, this part goes so smoothly you wonder what all the concern was about. But yes, occasionally it can be bad.
The other type of aggression is something you will almost certainly see, the pecking order. Chickens are flock animals. In order for them to get along smoothly as a group, they need to know where each and every one ranks in the social order. Often, this is determined by simple pecking. One chicken invades the personal space of another. The one that perceives itself to be higher in the social order pecks the other. The lower ranked chicken runs away. All is right and good in chicken society. If one does not run away, that is a challenge and they fight to determine which is higher ranked. Usually these fights are pretty brief, but occasionally they get serious. And if the lower ranked chicken cannot run away because they are housed so tightly together there is no place to go, it can get serious. This is why having lots of space is important. You have to give them enough room to get away from each other.
A twist to this is that a mature chicken always ranks higher in the pecking order than an immature chicken. Notice I’m talking about maturity, not age or size. A mature hen will sometimes get quite aggravated if an immature chicken gets too close. That’s why you often see younger chickens stay well away from older chickens when they are raised with the flock.
To increase your chances of a successful integration, try housing them within sight of each other for a week or so. Give them as much space as you can. You can increase your space by putting up extra perches they can stay on to stay out of the way or give them things to hide under or behind. Give them separate feeding and water stations so they don’t have to challenge the others when they want to eat or drink. Give them a separate place to sleep if they want it. The only place I have any real problems is on the roosts when they are settling in at night. Some of the hens may go out of their way to be brutal to the lower ranked chickens. I’ve had chicks that were used to sleeping on the roosts with the broody move to a safer place when the broody weans them.
My broodies prove to me that you can integrate younger chicks. They do it all the time. In the heat of last summer, one broody weaned her chicks at three weeks and they did OK. They were on the bottom of the pecking order so they avoided the older hens, but they had plenty of room to do that.
You may notice I frequently mention problems with hens. I’ve never had a problem with a dominant rooster in any of this, but it is almost always the hens that are the brutes. Occasionally an immature, non-dominant rooster will be a pest, but it is normally the hens that get vicious.
A story that might help demonstrate the difference in integration and pecking order. Several times, I’ve seen a two week old chick leave its mother’s protection and stand beside mature hens eating from the feed bowl. Occasionally the hens totally ignore the chick, but usually it does not take that long for one of the hens to peck the baby chick. That chick runs back to its Mommy as fast as tis little legs can go, peeping outrageously all the time. Mama ignores all this. That baby chick needs to learn proper chicken etiquette. But if the hen that does the pecking takes off after the chick, Mama takes great offense and teaches that older hen that the chick belongs in the flock.
In your specific situation, if you have plenty of room, you can probably manage the integration OK, though I’ll mention again that they are living animals and you can never be sure what will actually happen. If your space is anywhere tight, I’d wait and get them all at the same time.
