I've used grass clippings, with no bad effects. At first they picked through and ate what they wanted, then nested down in it. As long as it's dry, I don't see a problem. One thing I had to watch out for, though, was to make sure they weren't moldy and didn't have the chance to be. And offering grass clippings won't ruin their nutrition, if you're offering all the other good stuff. Once I had a bag full of grass that was full of earwigs, and I dumped it out in front of the chickens. They tore through that and ate every last bug! But it wasn't something I kept in a closed environment, since it was still damp. It was out in the open.
I would offer oyster shell if they're laying eggs... they'll eat what they need. So if they devour it, they're low on calcium, and if they hardly touch it, then their nutrition is on the right track. There's not much need for it if they're not laying, before they start creating egg shells. The right feed should give you the balance you need. Just make sure you also offer the right feed for their age, and don't offer too much "candy" like scratch grains. Chicken "candy" has the same effect as human-kid candy... it'll spoil their dinner.
I feel pine shavings are a matter of choice. I like to use straw because I can compost it for my garden. Straw stinks less but pine cleans up nicer. There is also a bedding made of recycled paper that is really good, and absorbs odors well.