Sure, the Constellation
may have been Decker's only command. If his higher rank needs to be explained at all (which, IMHO, it does not), we could go with the idea that Decker is about to retire and just wants that one command before he goes, or OTOH, he could have been awaiting more ships being assigned to his battle group later on. Either is likely.
is exactly right, though, the reason Decker was a commodore in terms of production was so that he would unequivocally outrank Kirk.