I don't think Suigetsu would win against Kisame, or rather against someone wielding Samehada. Given that science isn't too prevalent in Naruto, Suigetsu probably keeps his form through chakra control. If Samehada were to absorb enough of his chakra then he might not be able to keep a humanoid form, or at the very least he might lose the ability to wield a sword as large as the type the seven swordsmen seem to use.
If you consider how easily he tired out while walking, his stamina probably isn't as high as Kisame. Also given the nature of his body, he probably has a 'kamakaze' style of fighting against non-cannon fodder opponents, where he purposely takes a hit so he can get an opening to strike at the same time. Given Samehada's abilities, then he would be at an extreme disadvantage... or atleast he would if all my assumptions prove to be true.![]()