Hi, welcome to the forum from Louisiana, glad you joined,
First let's talk about quarantine associated with disease or parasites. When you go to school or work you could catch a disease if someone there has one. A lot of the time they don't have a disease but they certainly could. If you put two chickens together to live, if one has a disease the other is likely to catch it. Almost certain to, if one has a disease. Not all do. Same thing applies to parasites. And even if a human has a disease or parasite it rarely kills you. If you catch something it is usually more of an inconvenience that can be treated.
Some flocks, probably many flocks, have a parasite or disease that they have developed immunity to. Coccidiosis is a good example, basically a parasite that can cause a disease if the numbers of the parasite get out of control. But if the chicks or chickens are exposed to that parasite for two or three weeks without the numbers becoming overwhelming they develop immunity to that strain of that bug. They are still carriers and can spread the bug, but will not get sick, no matter how long they are quarantined. You will never know they have it. In that case quarantine doesn't mean anything. There will always be a risk when you merge chickens from different backgrounds, though I consider chicks form a hatchery to be as safe as you can get.
I'm not trying to downplay the importance of quarantine applied in the right circumstances. If a bird has been exposed to a new bird recently it could have caught a disease or have a parasite, then quarantine can save your flock. Flocks have been wiped out by bringing in diseases, usually not with parasites. If you are worried about something that the flock has developed flock immunity to it makes no difference.
Yours are not those circumstances. All of your chickens came from the same flock. They have already been exposed to what the others may have. Basically the two pullets have been in quarantine since you got them. The hatchery chicks should be very clean. With those chickens already there, any disease or parasite they have a flock immunity to (if any) is already at your place. That's the environment the new chicks are going into. They cannot avoid it.
I expose my chicks to their environment as soon as I can so they can start to work on building any flock immunities they may need. After two or three days in the brooder I feed them a small amount of dirt from the run where the adults are. This gets them any probiotics the adults have, gets grit in their system, and exposes them to anything they need to start working on the strengthen their immune system. I don't try to keep them in a sterile environment and then toss them into the environment they have to live in unprepared.
Stop worrying about quarantine. It would probably not have done you any good anyway, especially since yours have been in quarantine since they got there. For other people in other circumstances quarantine is highly recommended.
Single chickens can be hard to integrate. Different maturity levels make it harder. There are techniques to help, but what are you working with? What do your facilities (coop and run) look like? How big are they in feet or meters? Photos showing what they look like could really help. Do you free range or are they always confined? Where will your brooder be for those new hatchery chicks? Where do you live so we have an idea about your climate, your weather could be important wiht those new chicks. To me, it is a lot easier to give specific suggestions if I know what you are working with.
In general it helps to integrate if you house the chickens separate form each other but across wire so they can see each other, give them as much room as you can when they integrate, maybe improve the quality of what room you have by giving them clutter (stuff they can hide under, behind, or over), and don't force them to be together in a small space any more than you have to. Let them handle things at their pace. As you have seen it is not always this complicated, your 10 and 18 week olds did not have a problem. Each situation is different.