Hi
@davidsilber 
Welcome To BYC
What does the neck of the bald bum hen look like?
The rooster may be molting, but it looks like he is being plucked. On the hens feather loss on the back is looks like it is from mating, the bum feather loss is most likely feather picking as well.
Your coop size seems to be o.k. A "huge run" doesn't give us much information. Ideally birds confined to a run should have at least 10sq ft per bird of run space, the more the better.
As suggested by
@ChickNanny13 upping their protein won't hurt, provide them with some poultry vitamins once a week as well.
@KikisGirls suggested treating with a Permethrin based poultry dust or spray. This is also a good idea.
Watch your flock to see if you notice any aggression/bullying. Sometimes you won't see this during the day - roosting time is prime for feather picking. Add another roosting bar in the coop if picking is at night. Place some perches in the run (I use poplar saplings) so they can get off the ground and away from each other. Hang a cabbage, fruit, veggies for them to peck at. Even with only 7hens + rooster, there can be chasing/hoarding during treat/snack time - if you bring out a bowl of treats, etc. split in two - place bowls as far apart as you can.
For the bare bums, you can apply a little Blu Kote to help hide the redness and heal any irritated skin.
Keep us posted.