I am assuming (for now), a monopoly of power that enforces law and order and prevents crimes between C.U.s.

Any system becomes feasible once you assume a monopoly on power able to enforce an arbitrary law code. Of course, if you think about where the monopoly comes from you're back to a singleton scenario.

To the extent that CUs are made up of human-like entities (as opposed to e.g. more flexible intelligences that can scale to effectively use all their resources), one of the choices they need to make is how large an internal population to keep, where higher populations imply less resources per person (since the amount of resources per CU is constant).

Therefore, unless the high-internal-population CUs are rare, most of the human-level population will be in them, and won't have resources of the same level as the smaller numbers of people in low-population CUs.