For starters, the bloody mass from the vent that ultimately lead to the deceased chicken was most likely a vent prolapse. This can only sometimes be repaired, and often the chicken does not survive. Fortunately, it's just one of those things that can happen to a chicken, through no fault of your own, and cannot spread to and injure or kill any of the other birds in your flock.
In terms of the overall feather condition of the birds I think @Fishkeeper has some great starting questions because there are lots of things that can lead to tattered feathers.
Have you actually seen mites since you started with this new flock? We can guide you on treating mites if that is in fact what is going on.
Unfortunately mites are from your environment and transmitted by wild birds and can return to flocks periodically.
In terms of the overall feather condition of the birds I think @Fishkeeper has some great starting questions because there are lots of things that can lead to tattered feathers.
Have you actually seen mites since you started with this new flock? We can guide you on treating mites if that is in fact what is going on.
Unfortunately mites are from your environment and transmitted by wild birds and can return to flocks periodically.