It sounds like you are planning to have ALL 18 chickens living in this 4 x 8 foot coop, right? That's going to be mighty crowded! I think aart is right (6-8 chickens) unless they are going to have a big, secure and weather protected run attached. You get at least some snowy days, right?
Those are beautifully made nest boxes, but I agree that the chickens will need the floor space rather than all the boxes. I think I'd keep the set of six that you have on the small wall and take all the ones on the long wall out.
That will give you room for two roosts running the length of the coop, one on either side. Maybe you can get them a bit higher than the top set of nest boxes, but not much, to help alleviate roosting in the nest boxes (to avoid poopy eggs in the future). Most people use a 2x4 roost, either side up, depending on preference.
Now, I have bantams, and they would absolutely try to roost on those beams you have supporting the monitor. Not a lot of headroom there for full sized girls, but mine would cram themselves up there.