If you find yourself continually adding a couple birds here and there, it's best to build the quarantine cage right into the coop if you can. Something big enough for them to be comfortable in. 30 days is about how long it takes for them to show symptoms of disease, buys you time to really know if they are free from mites/lice/worms before putting them with your existing flock. I don't worry about disease as much as I worry about parasites personally, only because I've never encountered anything worse than a cold. But I have had to deal with scaly leg as well as mites in the past, and that sucked. I did things much differently with this new flock I have.
I had a big wooden box just hanging out in the yard, I don't know what purpose it had for the last owner. But I took it, flipped it on it's side, screwed some boards to the sides, and put the roosts those side boards, right on top of that box. The open end retains the floor space in the coop, and acts as a poop board, AND, when I see a bird I have to have, I have a piece of fencing that fits the opening perfect, and I throw the new birds in there. Right now my 3 babies are in there getting used to the coop and the big mean girls. But it's been way easier than digging out a cage, finding space, it's right there ready for a new bird or 3.
It also makes the introduction easier, because they could see and hear each other during that time, just no contact. At the 30 day mark, cut them loose in the evening near bedtime. They seem to take it better when they sleep with the new birds harmlessly. Course the new birds will likely sleep in their old quarters starting out.
Chickens are birds, and birds are sensitive to disease, drafts, parasites, all kinds of things. It really doesn't take much, just one sick bird that looked healthy on the 1st day, to doom the whole flock or half of it. In my case, battling bugs is a pain. I am still bug free in this flock because of being "paranoid".