Feathering sexing and development only works on lines that have been bred for that trait, and only within that line.
With feed store chicks, you get chicks that were hatched at different times from a broad range of lines. You never really know what the ages (and sometimes breeds) are in the bins.
Comb color and size is a much better indicator. If one of them begins to quickly develop comb, which pinks and reddens by 4 weeks of age, you've got yourself a roo. By 6 weeks of age, there will be a pretty substantial difference in comb size and color between roos and pullets for most breeds, but especially RIR/PR's.
Right now, nothing is screaming roo....time will tell.
Edited by Lady of McCamley - 4/30/16 at 12:36pm