I believe if I were to get the Maran and Cotswold I would need to isolate them from my current chicks because of potential illness but once this time has passed would they be able to join my existing chicks.
You are talking about quarantine, used for all kinds of different animals. It's a way to minimize the risk of one animal giving another animal a disease or parasite. Chicken diseases or parasites can be spread by them sharing food or water bowls, pecking at the ground where the other has pooped, by certain insect vectors like mosquitoes, grasshoppers, or grubs, or just floating in the air. You can track disease or parasites on your clothing, especially shoes, if you walk from one group to the other or use the same buckets to carry food or water or use the same food storage containers. The better you can isolate your groups the better your quarantine. Typically it is suggested you quarantine for about a month. Very few of us can actually do a good quarantine, we just don't have the room or facilities.
Another issue is that flocks can develop "flock immunities". By exposure they may develop an immunity to a certain disease or parasite but are still carriers and can pass that on to other chickens. No matter how long you quarantine they are unlikely to show any symptoms. This could be your current flock as well as the new one.
If the place you are getting them from has not exposed their flock to any new chickens for over a month to minimize their exposure to new diseases and parasites and you'd trust the owner to recognize a problem and tell you about it, they have essentially been in quarantine, just not on your property. That does require trust.
If the chicks are coming from an auction or maybe from a chicken show they have been exposed to other chickens and present more of a risk. That's where quarantine is more valuable. Lots of people get chickens from auctions or chicken swaps and don't quarantine. It is possible you can get something that will wipe out your flock but many don't have problems. When they do have problems it's usually something that can be treated, like mites, lice, or worms. Whether or not to quarantine and how you quarantine is a personal decision. I can't make that decision for you.
I would keep them side by side in separate enclosure before moving them together but how accepting are chicks of newbies?
It depends on the personalities of the individual chicks. Some are a lot more accepting than others. How much room they have can be very important. In general the more room they have the easier it is. The less age difference the better.
My goal in integration is that no one gets hurt. If they continue to be two different flocks with each staying by itself day and night and not intermingling, I'm happy as long as no one gets hurt. If you run into this situation they will become one flock when they all mature. If you insist that they all share a tight space before they are ready, it could get ugly.
One way chickens have learned to live together is that in case of conflict one runs away from the other. Then they stay away. This is why I keep talking about room. If they don't have room to get away when they run away and then avoid the stronger may continue attacking. If you see a chick standing over another laying on the ground and pecking at the head you need to stop that immediately. That's how chicks or chickens die.
It could get ugly anyway. Some chicks (boys or girls) have a personality that can only be described as a brute. There really aren't that many out there that will hunt other chicks to destroy them but there are some. The less space they have the more likely you are to see one of these.
It is possible that they will immediately join together when they are first introduced, especially with that small age difference. Just totally merge. It's possible the older will peck the younger if the younger invades their personal space. It usually doesn't take the younger very long to learn to avoid. This is if you don't do anything to prepare them to merge. You don't read about these success stories very much in here, usually just the problems, but this type of success is pretty common.
My suggestion is to try it and see what happens. Base what you do on what you see. You may need to use some of the standard tricks like housing them across wire, adding clutter to their area, and having separate food and water stations. Sometimes it is as easy as turning them loose, sometimes not. To me, one of the benefits of housing them across wire is that you are teaching the younger where to go to bed at night.
Good luck!
