Our small flock are "working girls" (yes, pun intended).
Our 8 Laying hens and two chicks free-range on most days over about an acre of land. They have 24/7 access to laying pellets in a 3 gallon feeder. Also have the chick-feed on the ground in their "room" of the coop. Depending on the weather, it needs to be refilled once or twice a week. They have one of those 24" hanging metal trays that I keep filled with oyster shell, so it's there when they want it. We also will put the empty egg-shells from the kitchen into the tray, and they will consume those first. I also put down a 1 pound coffee can of scratch three or four days a week. Our coop is set directly on the gound, which is mostly sand. Kitchen scraps go to the chickens as they happen. (I personally don't feed them meat products, but I know some people do.) I go thru about 25 pounds of scratch for every 50 pound sack of pellets.
Since your birds are much younger, I would give them the "grower feed" until they get close to laying age. I've been told that switching them too early will cause them problems, and that waiting until after they start laying to make the switch MIGHT also cause a problem. I don't know for sure, but I'm certain that others on the board can give you better information on when to make that switch. I think your best bet would be to consult your local County Extension Office, and they will know, or be able to find out, what's available locally, and what would be the best feed for your growing flock.
I too, have had the shell-less or brittle-shelled eggs. Once I started leaving the oyster shell out where they could get it as they pleased, I haven't had a single occurance. With sand floors in the coop, I don't need to provide grit, even for the chicks (or so I'm told).
I tried feeding only the laying pellets, and got 4-5 eggs a day, from 7 hens. Once I started supplimenting with scratch every other day, their production went up to 6-7 eggs a day, even the occasional 8 egg day. Not shabby for "low-maintenance" birds.
As for whether or not you can feed ONLY scratch, it would solely depend upon what the make-up of the scratch was. I've been told by the local feed store that the pre-mixed stuff they offer is fairly close to a complete diet for them. I haven't compared labels and such to find out for myself.
I can tell you this-- The amount of feed that they eat now that they free-range is about half of what they used when I had to keep them locked up in the run, their production is still at 6-7 eggs a day, and we now have 2-week old chicks ranging with the flock and not being bothered by the other hens.
Kathy