I realize you are in Maine and to you the winters are cold and nasty. You’re not a chicken. Cold itself is not your enemy. A cold wind hitting them and a lack of ventilation can make cold you enemy, however. As long as they are protected from a cold breeze they could do OK sleeping in trees, just like the wild birds that overwinter. Their down coat really makes a big difference. As Howard said, a lack of ventilation can create frostbite conditions, even in temperatures near freezing, let alone what you are likely to see. I’m not sure how much ventilation you have in there, you may have enough, maybe not. But if you start to see frostbite, people further north of you have solved frostbite problems by providing more ventilation up high. Maybe something to remember this winter if you see problems.
Silkies can’t fly so you have to make special accommodations for them as far as roosts and nests. I don’t think you are getting Silkies so your chickens should be able to fly up to the roosts even if they are fairly high. People have used ladder roosts forever, they can and do work. The dominant ones will sleep up high and there is a chance you will have drama at bedtime as they sort that out. And there is the chance that some will move to your nests to sleep instead of taking the lowest roosts, especially if the nests are higher. My roosts are all the same height and I still have some knocking each other off as the dominant ones get to where they want to sleep. A little disruption then is not a big deal, they sort it out.
I’m not a believer in magic numbers as far as space goes. Over time I’ve seen recommendations of anything from 1 square foot per chicken to 15 square feet per chicken on this forum, sometimes additional run space is called for, sometimes not. (In my opinion, 1 square feet is ridiculous and 15 is way overkill.) I’m an advocate of providing as much room as you reasonably can, but there is just no one number that works for everyone. To me, the more I crowd them the more likely I’ll have to deal with behavior problems, the less flexibility I have to deal with problems, and the harder I have to work. Most of these things concern you and your comfort and stress levels more than the chickens.
You may be fine with the room you have, you may find you have problems. You said you have a covered run. If you can block off some of the sides as well, you can create an area in the run that blocks the wind and keeps snow from accumulating. That is quality space. One way chickens have learned to live together in a flock is in case of conflict, the weaker runs away from the stronger or just avoids the to start with. By having area in the coop as well as outside the coop you have areas they can get away and avoid. Just putting some plastic sheeting on the side of the run could be a huge benefit.
I assume yours will all be about the same age. That helps keep drama down a lot. Different maturity levels can add a lot of stress in a flock, especially if space is tight. But you also have at least three males coming. You may or may not have problems due to that. I always recommend you keep as few males as you can and still meet your goals. That’s not because you are guaranteed problems with more males, just that problems are more likely. Your odds of having problems with males goes up as the space available goes down. Often, when they mature, multiple roosters claim a certain territory and have their own harem so they can avoid each other and cut down on conflict. You are not going to have much room for them to do that. I suggest you have a plan B handy. With three dozen chickens a place to isolate a chicken on short notice can come in handy anyway. With a coop that small you don’t have much room or flexibility, but you might want to plan on a separate isolation pen or small coop out in the run. That way you are prepared.
There is little doubt in my mind that those shelves and even the buckets on the shelves will become prized roosting areas. You might want to make sure they can’t poop in those buckets or knock them over by trying to roost (or just perch up there during the day).
With that many chickens in that small space, especially in winter with snow on the ground, you will probably find poop management is where you will have to work harder. I like droppings boards under my roosts to collect the poop so I can get it out of the coop. In one area I don’t have a droppings board though, I use plastic bins I got from
Walmart to set on the ground and collect the poop. I don’t use wire coverings though some people recommend them. Those plastic bins are pretty easy to just pick up and dump them. Just another idea.
As Howard said, plenty of people have done a lot worse than you and still been OK. I’m certainly not predicting disaster for you. You are tight enough you could have issues but maybe not. I do think adding some winterized room in your run to block wind and snow can help you a lot.
Good luck!