Course sand. Not the playground stuff but the stuff you mix with concrete.
Wood ash we get from our wood stove. We have a furnace but easier to just use the wood stove. And I like the wood heat better.
As for the ratio... nothing set in stone. About 60:40, sand:ash? We didn't keep track just put it in and mixed. The sand allows water to drain easily and just enough ash to give them some dust. Wish I could give you more details.
Yes, the grit and shell could go in regular feeders. I just used those small metal ones because they don't take up any room and they don't go through much. I've had to fill those metal feeders maybe once every couple of months.
Frequency of scraps... Again, really depends on your circumstances. During the spring, summer, and fall I won't buy them fruits and veggies; what they get is just excess from the garden and kitchen. During the winter I do give them something fresh once or twice a week. It isn't something you have to do. They are just spoiled and it is something we are able to do. It is more of a treat than a necessity. That head of lettuce I gave them this morning is only maybe a fifth eaten. They are ranging the yard and woods and ignoring it. Earlier this week, we had a couple of small zucchinis that were getting a bit soft. Wife is out of town on business and I don't eat them so I cut them up for the chickens.
Your mileage may vary. I'm pretty sure there are those on here that will say feeding them that way messes with the proper nutrition the chickens should be getting from the bagged chicken food. But last I checked, wild birds and critters don't live on bagged, nutritionally balanced diets. If I were raising large numbers of animals for profit, I'd want to be as efficient as possible, thus the bagged food. I'm not. I'm raising chickens for personal use; eggs, meat (eventually), and amusement. As long as they have free access to the basics (layer pellets, grit, oyster shells) adding other foods to their diets in moderation won't hurt them. I think the variety makes them happier, more content, and makes for better tasting eggs.
Hey knowing my redneck self I'll think of something. I can usually come up with anything to build if you hand me a hammer, zip ties, some wood & nails. My only problem is I tend to overthink something when I start a new project. I think about it so much until I'm so frustrated that I decide not to do it. I'm trying not to do that with this but it's knocking. what kind of sand do we use for the dust bath? And how much is wood ash or is it possible to find that for free somewhere? Could we put the grit & oyster shells (do the shells come in bags at TSC?) in regular chicken feeders? And how often should we give them heads of lettuce or cream corn, veggies/fruit? Should it be like an everyday thing at certain times?
it's similar to what others have said, put hardware cloth in the run or coop for a few days or weeks so they can see each other then take it out when they're older/seem to be getting along 
