We see Curzon in the orb vision Jadzia had in 'Emissary' when she first opens the case in the science lab.
And no, details about Sisko & Curzon's earliest meetin' hasn't been covered, not even in the books, though there was a "Lost Era" novel that came close (the one written by KRAD
But, really, is it so hard to see how a meetin' could happen? A diplomat needs to get somewhere, a ship is sent to serve as a taxi, and the captain assigns some junior officer to deal with the diplomat, thus Ensign Sisko met Ambassador Dax. He did tell Jennifer that he was had just graduated & was on his way to his first shipboard assignment, if ya take his first orb vision at face value of their initial conversation.
And in DS9 'The Forsaken', Sisko does give Bashir a similar assignment, pointin' out that he once had to do the same thing.