Regarding this, if the mimic act was a known or assumed ability of Kahless, that is, known to Kirk and Spock, our heroes should arguably have been wary of it. But perhaps the opponents weren't drawn from the minds of Kirk or Spock? Perhaps Kirk contributed Lincoln, and Spock contributed Surak, but it was Scotty
who contributed Kahless and all the misconceptions regarding him...
If, OTOH, Kahless was just a random collection of evil characteristics drawn from the mind of Kirk, then the Excalbians might have given him the ability for which, say, Li Quan was so infamous for, and Genghis Khan the ability that had made Jack the Ripper notorious. That would certainly keep our heroes from being prepared for the correct set of threats and dangers!
I'm still puzzled by the "You should stick to Star Trek" thing...?