I think it is pretty hard to over feed chickens... I suppose if they start feeling fat you can hold off. I have heard that Buff Orpingtons can get fat, but I doubt it. But if you are letting them get outside for some exercise, give them as many table scraps as you want. If you sprinkle to table scraps around the run, they will get extra exercise searching for scratching. We give our girls everything under the sun - grocery bags of overripe apples, leftover chicken carcasses after soup is made (yes, they love it), and buckets of unsaleable lettuce that the grocery store gives us. And we only have 13 birds!
If they have access to the outside, or you are only feeding bagged chicken feed, then you don't have to worry about grit. But since you are feeding scraps, it would be wise to feed them grit. If they have access to the outside, they will find their own grit. You can feed it in a separate container (but make sure their regular feed is full - if the feed gets empty they could revert to eating solely grit and that could be dangerous), or you could sprinkle it in their run for them to find on their own.