Tommboy,
Layena is considered a complete food for chickens, no need to add anything else. However, if you are concerned about weak egg shells then oyster shell is the thing to add to your girls' diet. I offer it to my girls, that way if they want it it's there for them, if they don't want it then they don't need it. No harm done
It's kinda like putting out salt blocks for horses.......they know what they need, and consume accordingly.
I always start my chicks with Start & Grow (medicated) and as they get older start offering PELLET Layena............less waste than crumble Layena. It usually takes them a little while before they take to the pellet shape, but they grow to like it and it saves on waste.