A never ending process for sure. Each situation is different because each individual chicken has its own personality. You can never be sure how they will react. That's what we get for dealing with living animals.
Quarantine shows that variability. It's possible any flock can have diseases or parasites that they can pass onto another chicken but never show symptoms themselves as they can develop immunity to many things. Your hen might have that, the cockerel might have that, or the new ones you get could have that. Or they may have nothing. If they have anything it is most likely a parasite like mites or worms that you can treat if they show up, not a big deal. But occasionally (rarely but it happens) you can have a disease or parasite that wipes out your flock. You just don't know what you have until something shows up, if it does. Many people don't pay any attention to quarantine and don't have any major problems but there is always the chance they could. Some diseases and parasites can travel on the wind or be spread by insects or other animals like wild birds. Many people on this forum think they are quarantining but really don't have enough separation to have an effective quarantine, yet they seldom have problems. I cannot tell you that you have to quarantine but I also cannot tell you what will happen if you don't.
The reason I suggest you get hens that are already laying or are old enough to lay is that they will stand up to the other chickens and take their place in the pecking order. Immature pullets will not, they will just run away. Some people go through a long drawn-out integration process. I certainly suggest that if you have juveniles involved. With your situation and with adults I'd suggest just turning them loose with the others giving them as much space as you can. They will establish the pecking order. That may be pretty peaceful, it may involve fighting. If they have a lot of room any fighting is generally over with pretty quickly. But you need to pay attention. Sometimes it can go on a long time or one may become injured so you need to intervene. Again, I cannot tell you what will happen, it depends on their individual personalities. You may still have to isolate them and house them side by side for a while, but I'd try just turning them loose while they have enough daylight left to sort things out.