I think Kisame is too overpowering to be defeated by what Mei is shown. Although I am waiting to see if acidic release will be more than just acidic mist (which it should be, than the whole KG is like 1 powerful technique)
Hahaha. Yeah, Kisame wins quite handily. imo he would make a hundred times better Mizukage (though I'm sure his administration skills would leave something to be desired). Samehada added just means Mei has almost no chance of beating Kisame instead of a low chance.