If he wins all those original 7 swordsmen, he would easily beat Kisame.
Kisame is on a whole different level than any Swordsman we've seen so far. Killer Bee is a strong guy and Kisame nearly beat him without actually trying to beat him. (He was just trying to infiltrate the Cloud)
Kisame has chakra liked a tailed beast, plus Samehada and his jutsu. He is a monster and a tough one. Kakashi doesn't have the best chakra supply ever. He'd use it up with Lightening Jutsu, aswell as maybe copying some of Kisame's shark jutsus, like he did when they first met.
His MS would be useful in the battle, but Kisame is fast and has a chakra draining sword.
I love Kakashi. He is awesome and super strong, but Kisame is in the same league as Killer Bee, and I don't think Kakashi could beat Bee.
Kisame wins in my opinion, but the fight would be amazing. I can see Kakashi almost winning alot of times, but Kisame's massive chakra compared to Kakashi's low chakra is a major difference. Also, Kakashi's amazing ninja skills could disarm Kisame and Samehada, but he has to get in close range. Also, Kisame could just fuse with the sword to avoid that. Who knows.
Kisame wins in my opinion though.