Re: does a starfleet ship always have a CO with the rank of captain?
I think being a starship captain carries an enormous responsibility--at any given time, entire worlds and millions (if not sometimes billions) of lives could depend on the decisions he or she makes. So regardless of the size of the vessel, the billet for the commander of a frontline Starfleet vessel could indeed warrant a captain. The exception may be with small support craft or small starships assigned to a starbase (in the latter scenario, the ship would perhaps operate under the authority of the starbase CO, even if a lower-ranked officer was in the center seat).
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