During molting, I like to use Feather Fixer. It really speeds up the process.
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Could switch everyone to an 'all flock' feed, serve whatever calcium supplement you prefer (egg shell, oyster shell, etc.) on the side, and can even grind it fine to be used as a non-medicated chick starter. Truly is an all purpose feed that won't have you skimping on nutrition or overdosing the calcium for birds that are not actively laying such as unstarted pullets, chicks, cockerels and roosters. Birds seem to realize that too much calcium is bad for them, and avoid it when they don't need it, so don't mix it into the feed, just serve it on the side.
At 15 weeks they're close enough to laying that you can feed layer without worry. I typically feed medicated Starter until 12 weeks and then Grower until they actually lay an egg. I don't usually feed Layer, preferring an All Flock with free choice oyster shell. Some of my hens had issues on Layer, plus I kept a rooster in with them.
I too prefer a flockraiser type feed, with free choice oyster shell on the side.
I'd just feed everyone an all flock or a grower for the time being and offer oyster shell on the side for the layers to peck out when they want.
Not all all flocknis crumbles
I don't feed layers feed anymore, but I did with my first Flock of Golden Comets.(the youngest ones in that group are 15 week old barred rocks) so can I move them out now and feed layer?