I have both large chickens and full sized goats, the only way to keep the goats out of the chicken feed is to keep it inside the coop, and the chickens access the coop by a small door (10"x12") that the goats can't reach the grain or squeeze through.
Very nice coop you have, I have a similar style just a little bigger (8 x 12). I am in Vermont so I also deal with a cold winter. I have raised chickens for decades successfully without using heat or insulation. I close all the windows, but have an additional 6 inch strip of ventilation just...
I have the same size coop for 28 chickens. This spring I ordered 25 chicks to raise, and tried a completely different brooding setup for me which was a total success (100% survival with no illnesses). I brooded them in a 2 1/2 x 3 1/2 Foot wood box right in the coop using 2 mama heating pads on...
I also live in VT, have raised chickens for over 30 years and have never insulated or heated my chicken coop. There are no drafts that hit the chickens when it is closed up for the night, although there is ventilation from under the open rafters in the back of the roof all year long.