I was never a fan of feeding layer rations to males. But I only had a few birds before. Now with 19 birds at several months old and 30 week olds, I think I will just feed layer once everyone is at the right age. The week olds are in and will remain in a seperate area from the older birds so once my ducks and hens start laying, I'm just going to switch them to layer. I just can't see spending the extra money on enough oyster shell for all of them. I do give them (2 older hens) crushed, cleaned, dried egg shells but I don't eat that many eggs that I could supply them to everyone.
That being said I would NOT give it to younger birds (< 4 months ). I plan on feeding pellets, not crumbs so it would be difficult for young birds to eat it anyway.