You said you have the material to build the new coop and run. Couldn't you use some of that material to build a temporary small(ish) coop/run (tractor?) for the new chickens with some of that material? Then you could take care of both the quarantine and the space issue while you build the new. I'm new to keeping chickens, but I personally would quarantine. I would be devastated if something happened to my flock due to my own negligence (even if they "look clean"). Not that you HAVE to do what I would, just stating that is what I would do.