Feed what is easiest for you. Chickens prefer? My chickens love to eat, crumbs- pellets makes no never mind. I can garauntee if they get hungry they will eat whatever is presented. They will not starve to death when shown pellets. It would have to be some real stupid chickens to starve themselves.
Bantams, heavies, broilers, whatever they ain't gonna choke on a pellet. You ever see what a free range chicken gets down into his gullet? Rocks, marbles, glass, nails, kids toys, a pellet won't be a problem.
Supplement oyster shell, why? If it is laying pellets it has all the Ca needed. To break it down? Pellets are compressed crumbs, they ain't glued. The grain portion of the pellet is ground just as fine as the grain in a crumb. If the chickens are not raised on the ground or free ranged you may want to supplement GRIT not oyster shell. But that requirement would be equally desired with the use of crumbs and pellets.