Your coop is not big enough to put in a dog cage with enough room to comfortably keep two hens. And, as ChickenCanoe stated, that's not true quarantine anyway. To truly quarantine, you need to keep them hundreds of yards apart, plus change shoes and clothes every time you go from one coop to the other. People will keep them separated in a "see but don't touch" situation hoping for easier integration. This may work, but once they are in the same space, there will still likely be squabbles so they can sort out their pecking order. This happens with chickens. It's hard to avoid. If you are not worried about quarantining them, I'd just add the extras at night so they wake up together. This does not mean all will be peaceful in the coop. I just think it's less stressful for them to wake up in a new place rather than be tossed into a coop. I could be wrong... it's just my opinion.
PS - if your coop is adequately ventilated, they really don't need the heat lamp. I'm in MN, so I understand cold weather. They'll be fine, really. Especially with that many chickens in that small of a coop. Their body heat will suffice.