What do you mean by shell grit? I'm not familiar with that term.
Different people use different things. No matter what you use somebody will tell you it is unsuitable. We all have different conditions and management styles so sometimes certain things work better than others. Sometimes trial and error is involved to see what works best for you. I like something inexpensive and available.
For nests people might use wood shavings, hay, straw, carpet, wood chips, sand, rags, shredded paper, or feed bags (plastic or paper). In the Gulf South of the USA people might use Spanish moss. I cut tall grass for areas I don't generally mow or weed eat and dry that.
For the coop floor people use wood shavings, wood chips, hay, straw, sand, dirt, or shredded paper. Some use a linoleum rug, usually under a bedding material. Some people turn their coop floor into a compost pile (called the Deep Litter Method). In addition to one or more of the bedding materials they toss in kitchen or garden wastes. This is tricky as you want the compost to be damp enough for the microbes that eat it to live and reproduce but if it gets too wet it can turn slimy and really stink. I use wood shavings on a dirt floor. The coop floor is built up with dirt a bit to keep water out. It is to dry for the deep litter method.
Basically when it gets dark they should go to sleep. When it gets light they should wake up. I'd think the coop is much bigger than the brooder was so they should behave quite well.
Where they sleep is the question. At 9 weeks old they might go up to roosts or high spots. They may huddle together on the coop floor. I'm OK with either one. They mature at different rates. When they are ready to go to the roosts they will. It did not hurt them to sleep on the brooder floor, it will not hurt them to sleep on the coop floor.
