Herne Hill Saturday Training

RPM, the problem is when stronger riders let a gap develop, they have no trouble putting on the gas and closing it. but if a weaker rider is behind they are then left out in the wind and may not be able to. it also causes concertina-ing...

What causes the concertina effect is people not controlling their speed smoothly. Not necessarily the size of the gaps.

Actually, you will get more concertina-ing when people kick back because they are too close to the rider in front and have no space left to control their speed.
A wheel length to 1/2 a bike is actually better for these types of session with riders of mixed ability. Learning to manage a gap at 1/2 a bike will teach people how to ride in a group far better than believing they must be 'on the wheel' immediately. That can come later.

Even the Olympic team pursuit team have changed the 'fag-paper' gap to about 8-10" now so they have room to control their speed.