Don´t get hung up on rank, so what if he was almost hokage, sasuke is a genin and he took on the 5 kages, he would pimp slap kakashi, naruto the same thing, his clones are taking out kages and s-class ninja in 1 or 2 moves while him being genin, kabuto beat tsunade in part 1.
Now, even if kakashi uses mangekyou and kisame loses samehada, he can still beat kakashi, kakashi can copy all he wants but he doesn´t have the chakra or the power kisame has.
Kisame makes a humungous water-shark jutsu, kakashi copies and he still gets ovepowered, genjutsu won´t work that well and taijutsu is just suicide.
Besides, kakashi will be weakened from using mangekyou.
Kisame still wins.