Welcome to BYC. YOu may get better response to your questions if you start a new thread. This one is OLD!, and folks often don't respond to a thread if they see an old date on the first post. Any how, IMO, grit should be offered soon after hatch. Birds have no teeth, and they were created with a gizzard, which requires grit to function. If your sandy soil had hard bits of stone in it, it should be fine. There are some areas that simply have NO stones at all (consisting of fine silt). So, it depends on the make up of your soil. Consider this: wild birds are able to keep their gizzards stocked with grit, but they have a large area to forage over, and are not limited to your back yard. So, you'll have to assess what you have available and make your choice. I keep a bag of grit on hand. While I live in a very rocky area, my ground is frozen solid from November through mid April. So, my flock really appreciate a dish of grit in the winter.
I would caution you regarding that feed that you are still using. It is now 4 months old. A poultry nutrition expert in the flock management book written by Harvey Ussery cautions that feed 6 weeks beyond it's mill date is loosing nutrients, and on it's way to becoming rancid.