Hope that helps. The recommendations for how many birds a coop will hold are often way over estimated, especially with pre-builts. I've seen them claim to be large enough for 6-8 birds and I wouldn't put more than 2 in there. The behavior of the birds is the most important factor. One...
It looks more like feather picking, probably happening on the roosts. You may need to add some roost space. I would do some observing, see if you can see what's going on. The feathers may not grow back until they molt. Exposed skin often looks redder.