If there isn't enough room for two roost bars once you move them away from the vents, then have just one roost bar, cover up the vents on the access door, put the partition up and have bigger vents on the other side. The chicks will be small at first so they'll fit on one roost bar. As they start getting bigger, start getting rid of them - rehoming, butchering, whatever - so you end up with enough birds to fit on one roost bar. Then, next year, when the weather warms up and you have more daylight, time, and savings, you build out that extension and get more birds then. But for now, if you can't give X number of chickens good living conditions, it's better to scale down and have fewer, but properly housed chickens, as opposed to a lot of frostbitten ones. And if you don't think you can do the butchering yourself, see if you can pay somebody else to do it. It's a quick job and a quick buck for somebody who's willing to do it. There's gotta be somebody out there, maybe the local butcher or a farmer or something.