I would expect the KCs to start the ball rolling, then the Ancona, and finally the Pekins and Rouen. And ducks can make liars out of us!
Let's see - if yours were hatched 2/28/2015, 20 weeks average first egg for Khaki Campbells would be, umm, sometime late June or July. When they start trying to mate, that's when I would set out the free choice oyster shell. When most are laying, I would then switch to layer feed.