It is very important to quarantine and to take it very seriously.
Believe me, take it from someone who had to put down their entire flock...
Even when my new chickens were quarantined they still got my other chickens sick.
If it was as easy as treating all the chickens for something they could get, then it wouldn't be so much of an issue.
The problem is that their are disease out their like Corzya, Mycoplasma, Mareks, ILT.
Lots of these disease will make your chickens carriers for life once they are infected/exposed and they are very contagious.
So you just can't put all the chickens together and give them medicine because it could be something that isn't curable or treatable.
Your quarantine area should not be anywhere close to where your flock is.
Some diseases can travel by air and soil transmission.
It is always very important to constantly sanitize after working with the new birds.
Diseases can be carried on your skin and clothes.
I would work with your flock first then work with the quarantined birds second.
Because there is so much of a risk buying adult birds, i have choose to only hatch out eggs or buying hatchery chicks.
So if you don't want to quarantine, just get some hatchery chicks or hatching eggs.
Losing my entire flock was heartbreaking and now i'm totally paranoid.
I have to oxine/bleach out there coop, put hydrated lime in the run, and wait a few months before i can have chickens again.
One little mistake or underestimating a disease can ruin everything.
Please understand i am saying all this with the best intentions.
I am not trying to scare you, or pick on you, or anything like that, so please don't take this the wrong way.
But i just never want anyone to have to go through what i went through.
If you can prevent something horrible like that from happening then i think people should know.
Anyway, good luck with your future flock!