Cohort commander?

[Nathan Ross]

In general, cohorts of Roman citizens had tribunes in command.

This is actually an interesting point. If we accept, following Fronto, that a centurion was equal to an (auxiliary) cohort prefect, and that citizen auxiliary cohorts were commanded by men of the next rank (ie tribunes), then officially speaking (!) a centurion of any grade may have lacked the authority to command a legionary cohort. Only, perhaps, if the cohort was detached and the centurion in question given the temporary position of praepositus, which would raise his acting rank slightly?

Perhaps this is the reasoning behind those strangely composed vexillations, with men drawn from different centuriae and different cohorts, all banded together? That way, they wouldn't officially be a cohort, so could be placed under the command of a mere centurion! This seems an incredibly fussy and legalistic distinction, but the Romans were rather given to fussy legalisms...