I guess I am a bit confused maybe.
If I add crushed oyster shell into the grower pellets is that the same as feeding them layer pellets? Is the oyster shell for added calcium or is it like a grit?
What would you recommend?
Oyster shell is provided to give extra calcium to laying birds. Younger birds shouldn't be given oyster shell. Oyster shell shouldn't be added to the feed, but given separately. The laying hens will eat only what they need, so if you add it to their feed, they may bill out and waste oyster shell and/or feed.
Granite grit is used as grit to help male and female birds of all ages digest their food.
So according to you then, my best option for feeding hens & roosters is to feed finisher to all and offer hens oyster shell.
and that's sufficient for the laying hens?
If you have mixed sexes, and ages in the same pen, it is the best way I have found to not feed the extra calcium to young birds, and roosters that don't need it. If you provide oyster shells, and give them plenty of sunlight, they will lay nice thick egg shells for you without layer feed.
I keep my very young birds (up to three months) separate (so they are unable to get the feed or supplements offered to the other birds)... I also have silkies which tend to start laying much later than other birds.
After that I just offer the layer feed in a separate feeder (in my case mixed cracked grains are mixed in / figured in wiht the nutritional profile so I would not want them diluting their feed by picking those out which is why I separate the first two/three months) .... I have never seen my roos or young birds eat the layer pellets. My adult girls do though and in addition to that eat the oystershell.
It is not an "either/or" situation IMHO as calcium absorption can be influenced by many things and the oystershell is there for those times the girls need that extra (despite having layer pellets available)