Actually watching the interactions and how the hierachy develops over time is fascinating.
I'm convinced that I have separate groups that live in different zones of the tank and only come together at feeding or breeding time before going back to their own areas. And yes there is a group living in the back. This does not seem to be related to size because there is a full range of sizes in each area.
I can't actually guarantee they always go back to the same areas because mostly I can't identify individual shrimp - but the few that I can (individual markings and scars) do seem to always be in the same areas.