My nesting boxes are pretty high, though the roosts are (most of them) higher. I have one little pullet who likes to sleep in one of the high nests. I let her, because she’s the smallest and the others drive her away. I just flick the poops out when I open the doors for them in the morning. It’s not bad.
If you don’t want them in the boxes at night, you can hang a bit of old tarp or similar over the entrance to them and roll it up like a tent flap during the day. For eight, two boxes should be enough.
The roosts can be as high or low as they need to be. The chickens will roost in the highest spots they can reach, most dominant—highest spots. You’re supposed to have a foot of roost per standard-sized bird. I have 17 birds and 12’ of roost space in a ladder roost 4’ wide. They all (except the little one) mash up on the top two rungs—8’. So, 6” per chicken. They only use the bottom rung for hopping up and down.
If you had too small a space for roosts, they’d roost on the floor and be happy anyway. They like roosts though, and being high up provides a little safety, too.
Since you’re in Indiana, it would be helpful to put the broad side of the lumber up, so they can keep their toes from frostbite in winter (and “spring”) by sitting on them.
I would not run the roosts longways without adding a support in the middle of them. They’ll eventually sag if you do.
