You didn't mention age. Do you know about how old they are?
I would begin with either a chick starter, or flock grower feed with 18% protein, and free choice oyster shell on the side. Fill their feeders, and let them eat as much as they want. Don't worry about impacted crop. Even newly hatched chicks don't need grit when all they're eating is feed, because it won't impact their crops. Don't try to rush it. You are wanting to build muscle, not just fat. Don't give them anything but feed for now, because it meets 100% of their balanced dietary needs, and treats don't. When they attain their weight, and are in good health, then you may add the occasional treat.
I would begin with either a chick starter, or flock grower feed with 18% protein, and free choice oyster shell on the side. Fill their feeders, and let them eat as much as they want. Don't worry about impacted crop. Even newly hatched chicks don't need grit when all they're eating is feed, because it won't impact their crops. Don't try to rush it. You are wanting to build muscle, not just fat. Don't give them anything but feed for now, because it meets 100% of their balanced dietary needs, and treats don't. When they attain their weight, and are in good health, then you may add the occasional treat.