Scratch should only be given as a treat, maybe one handful for 6 or 8 chickens, once a day. It does not have nearly enough protein for growth or laying. This may be your whole problem. You need to give them regular chicken food, probably either grower or layer, depending on age. And you need to supplement protein at this point for a while as well, if scratch has been their main feed. Yogurt, canned mackerel, cottage cheese, meat scraps, cooked hamburger, cooked cheap liver... whatever. Plus, it takes protein to grow new feathers. Veggies are great when they are on a balanced feed, but do not usually provide protein, and it certainly sounds to me like they need a good protein boost at this point.
Glad to hear you will do the Sevin.
That is hilarious to me, that the buff orps boss the guineas around! Glad to hear it. We never know -- breeds do have tendencies, but chickens are such individuals....
Does not sound to me like you have much of a rat/mouse problem, but it is always good to monitor this....