Scratch - This is usually a mixture of grains, though sometimes people just use straight corn, that is given to the chickens as a treat. It does not have the same balance of nutrients that they need, but corn often makes up a large portion of regular chicken feed. Other things are mixed in the feed in a the correct proportion to provide a proper balance of nutrients. There is nothing wrong with giving treats as long as you don't give them enough to upset their "balanced" diet. Usually if they can clean it up in 10 to 20 minutes, it will not be a problem at all. Look on it as feeding a child candy. It is nice to let them experience some of the finer things in life, but don't feed them so much candy they don't eat their regular meals.
Grit - Chickens do not have teeth to chew up their food, but they eat things that need to be chewed up. They solve this by using pebbles and sand in thier gizzard to grind up their food. They can usually find stuff to use as grit if they have access to the ground, but if they do not have access to the ground, you can get granite grit at the feed store and offer that to them on the side. You can also just rake up small gravel from a gravel road or some similar place, but if your roads are salted in the winter, you should not do this. Their system cannot handle the salt. The size they use is anything from about the size of a green pea down to coarse sand. An easy thing to do, especially if you have a wet spot in your run, is to get a bag of pea gravel and dump that in their run. It will help keep the footing firmer and they can find plenty of stuff to use as grit.
Since they are laying age, not chicks, providing oyster shell on the side is fine. The extra calcium in oyster shell can damage growing chicks' kidneys or cause bone deformation. But since yours are grown and laying, this is not a concern. They need calcium for their egg shell development.
You should be feeding them Layer, it does not matter if that is crumbles or pellets. Layer has all the nutrients they need to stay healthy and lay well. It also has extra calcium in it. They will probably get enough calcium from the Layer, but it does not hurt to have oyster shell on the side.