Cohort commander?

[popularis]

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...

I've had the same thought as well, with regards to praefecti in command of Roman legionary troops in the Republic! In the end, I was worried that I was putting the cart before the horse: if you have an irregularly comprised unit, you can't simply drag tribunes away from their normal duties to command them, so you have to appoint a prefect.

I worry, too, about fussy and legalistic distinctions. Not because they didn't exist (hello there, imperium, I'm looking at you!), but because I think it's very tempting for us to apply them in order to fill in the gaps in an otherwise fairly chaotic collection of evidence. Sometimes it's even more tempting to invent them: I used to think that equestrians during the Republic never served as centurions, but I realise that I've spent a lot of time explaining away apparent 'exceptions', and now I'm not so sure.

I'd be tempted to split the question into two: 'is there any specific reason why a centurion could not be placed in command of a body of citizen troops?' and 'is there any reason why a centurion might be preferred above an equestrian-ranked man (and spare ones usually existed, in the commander's cohors) for such a command?'. I don't have any answers, mind you - not for the imperial period at least!