Yeah, it will throw them off a bit, but they'll adjust pretty quickly.
You'll have to think carefully about where to locate the roosts because of the narrow aspect (4'?) of your coop.
They need room to fly/jump down and land safely without crashing into something....
.......you might want to...
Just because they were gathered together on the floor doesn't mean they were cold, they find comfort and security in touching at times, especially when sleeping.
The chatter after lights out was not because they were 'scared' of the dark, but the sudden darkness alarmed them, they can't see...
Yeah, that cute little pullet poop is nothing compared to a full grown bird.
Even with an immaculate coop, ventilation is important.
When it gets hot down there, it's going to be even more important.
How big is your coop(feet by feet)?
Are there lots of windows for light and ventilation?
Agrees that a battery powered push light is a good idea...if you still need to use a light.
They are young and may take some time(yes, weeks and weeks) to grow good habits.