Perhaps pick another wall and use 2x4s to attach another roost, coming from the wall and above the floor of the coop, maybe at "adult chicken height". Agree that the interior space is inadequate for your flock, but I wouldn't worry about their sleeping arrangements until you have the chance to increase the space. Everyone will eventually sort themselves out, provided there will be space and privacy for sleeping. If my girls want to sleep in their nest boxes I let them--it's their choice. I clean my coop and my run daily anyway so no big deal about poop in a nest box--I'll just pick it out. And remember: chickens are the pet that poops breakfast! 
