Re: What do we know about the years between ST5 and ST6?
My impression was never that McCoy didn't know Sulu was a Captain, but that he didn't know why all the old Enterprise command crew had been assembled except for him and--as Kirk asks--Spock.
"Canon is only important to certain people because they have to cling to their knowledge of the minutiae. Open your mind! Be a Star Trek fan and open your mind and say, 'Where does Star Trek want to take me now'." - Leonard Nimoy