Yes, I believe you can give too much scratch. I was recently on vacation and my neighbor, who loves my chickens, fed them scratch with abandon. I came home to a thick layer of it on the ground of their run and pale, claylike poops from the girls. (I think that he thinks of them as his grandchildren and feeds them junk food.)
Scratch is not as nutritionally complete as their commercial food; having said this there's a recent post about raising chickens without commercial feed that's pretty interesting and informative. I choose to feed mine commercial feed. I give them a small amount of scratch, 3/4 cup, split between six hens before bed so they have a full crop at bedtime. My scratch is homemade and I don't have much corn. It does add fat and fat in laying hens is not good.
I think in the winter it's a little more important to increase their protein intake because they don't have access to the bugs and things that they do in the summer. This can be done by getting meal worms or crickets from the pet store, changing their feed to one of higher protein or giving them a little bit of raw meat like hamburger. Also, some fish foods are good. There's a need for caution using things like cat food because of the salt. But don't give them too much high protein food; they're omivores.
It's possible to make your own mixture of scratch that they'll eat, or throw some safflower or sunflower seeds out for them. These are pretty nutritious. Hulled are expensive, but chickens have no trouble digesting BOSS (Black Oil Sunflower Seeds) with the shell. You can also give them left over salad fixings or get throw away produce for them from the grocery store. Also, you can get alfalfa cubes, soak them in water until soft and feed these for extra nutrition.
It's true that the chickens of my dad's childhood ate leftovers and whatever they could catch but it's easier for me to feed them a scientifically sound diet because I worry about illnesses and the like. Also, yesterday I got four eggs from four layers and it's dark and cold here in Colorado.
Mary