Just remember that chickens are one of the oldest of domesticated animals, they can and will eat almost anything and thrive off it. However thin modern plastics are a danger so keep them away.
If your chickens are not cage kept and allowed out in the yard or free range you will not need any supplements to a 50/50 mix of "All Flock" and "Scratch". However if your flock are an industrial laying breed (6 eggs per week) than you will have to supplement calcium as those birds are genetically engineered to over-produce and it wears out their bodies.
Again "Grit" is just sand and gravel, they use it like we use molars but it's in their craw and it grinds food to start digestion.
Most importantly - chickens are like toddlers... ...anything and everything they can find eventually ends up in their mouths.