Technically Dax get promoted to Captain... Ezri that is, in the books, several years later.
I was going to make that point.
RE: Commodore
As I understand it from various sources, Commodore still exists although it's not an official rank, its more like an honorary rank given to someone who needs temporary authority, but is not necessarily in the Star Fleet chain of command.
For instance, if Picard would come back to Star Fleet on a temporary basis to take over command on something, they would temporarily give him the rank of Commodore. He might also be granted the rank in the event that various diplomatic missions required him to be in charge.
RE: Rank required to command
Captain is absolutely not a required rank to command a ship. Certain levels of Star Ships have certain dispersion of ranks.
Voyager demonstrates this a bit. Chakotay is the first officer and up until Voyager premiered, we always saw the first officer of a star ship hold the rank of commander. In Voyager, the first officer holds the rank of Lt Cmdr.
Likewise, up until Voyager we always saw the Operations Manager hold a rank of at least lieutenant where as Harry Kim is stuck at ensign for seven years because the post he holds on that ship only requires an ensign.
In Star Trek the magazine they revealed that Defiant class ships only require the rank of Lt. Cmdr. to be the full time commander. Other escort ships like the Streamrunner or Akira classes also don't require the rank of captain to command it. Likewise, the Nova and Intrepid classes only require the rank of Commander. Janeway and Ransom were captains of their respective ships because they took out one of the prototypes of their respective classes. They want a captain (or an Admiral) in charge of shake downs for new ship designs.
RE: Dax not on command track
Don't forget that Dax was technically the XO of Star Fleet operations on DS9 at the start of the series. In any operation that was strictly Star Fleet, she was the direct next in charge after Sisko.
When Worf came on board during Way of the Warrior in season 4, I'm of the impression he takes over that role. There are a few episodes where he says to Sisko "I wouldn't be good as your XO if I didn't question X".
OBVIOUSLY since Worf wasn't around after Call to Arms, serving as Martok's XO, Star Fleet seemingly still recognizes Dax as a command level.
Like Spock, Beverly and Diana (all who also where a blue uniform), Dax holds a command position.
Also, its worth noting that Star Fleet recognizes the immense benefit of being a joined Trill as Sisko convinces Star Fleet to give her the rank of lieutenant because of her past life experiences.