I'm kind of thinking we either won't see Kotetsu's sixth Pokémon in his battle against Ash or we won't see it in this episode.
Now if Kotetsu does defeat Ash and we don't see it, I am hoping Kotetsu will use his sixth Pokémon in his next round battle.
Fact 1: Cameron/Kotetsu is not disliked by everybody, stop saying he is, there are people in the fandom who do like him. I am one of those people.
Fact 2: The majority of people who like unpopular characters are always the most nicest and sensible and, therefore, the best people of the fandom.