Cohort commander?

[Frank]

Could not centurio decimus mean the tenth hastati posterior century and not the tenth cohort?  

Actually "decimus hastatus posterior" just means the "10th (centurio) Hastatus posterior". Not the "Hastatus posterior of the 10th cohort" or "of the 10th century of hastati posteriores". If I am not fully mistaken linguistically.

A commander of a century which belongs to the 10th cohort. And to the 10th maniple of the hastati in earlier times. So finally it probably meant both. At least after the cohort structure became common. As you know, romans were very traditional, if it comes to legions and naming.

So your conclusion about the naming of the 6 x 10 centuries sounds correct. But it is also nothing new. A roman would perhaps call it nitpicking, because he did not differentiate between "Hastatus posterior of the 10th cohort", "Centurio posterior of the 10th maniple of Hastati" and "Centurio of the 10th posterior century of the Hastati". It is just all the same for him, as far as I know.