They eat the layer feed too. As long as they are 18-20 weeks old at least, otherwise the extra calcium would damage them and they might have later health issues.
For adult roosters, if you happen to let them range freely, the roo will be able to get enough other proteins (via bugs) and greens and all to supplement the feed, reducing the calcium "impact" that some folks are concerned about with their roosters.
However, I don't feed layer feed to my flock. I have too many different ages of chickens, so I feed EVERYBODY a general purpose, non-medicated feed (grower/finisher, similar to FlockRaiser). I supply crushed oyster shell free choice for the layers, in an empty tuna can nailed to a coop wall. I also toss out oyster shell in one particular spot in the yard, so there's more than one place for them to find it.