I just can't picture Kor in the Trouble with Tribbles....
Given Kor's obvious intelligence and sense of humour seen in "Errand Of Mercy" I can easily see easily see Kor in "The Trouble With Tribbles" and John Colicos easily pulling it off.
Indeed they wanted Kor to return for "The Trouble With Tribbles," but Colicos was unavailable. Pity. I liked William Campbell as Trelane, but I just rolled my eyes seeing him as Koloth.