The chickens can use the duck step just fine. It is easier for them to grasp the step with their claws, one of the reasons we use a narrow treadle, but they can grip the duck step rough plywood surface just fine. One feeder is plenty for around ten to twelve birds. That would feed them for four days if it was a small feeder, ten days for the medium.
Rojomarz did great in pointing out the one drawback, with the duck step the feeder is less rat proof because the rodents can get closer to the feed if enough of them gang up. Kind of makes it like the other treadle feeders that all have the big honking wide steps. But we still have the only feeder that has a spring loaded door that stops a rat from just pushing the door open and chowing down. You can start with the duck step pushed forward, drill a couple extra holes and move it back an inch or so each week till you have a good six inches between the duck step front edge and the feeder itself to increase the resistance to rats if you have an infestation.
