It doesn't seem like good writing for Ash to just bring his reserves back out of nowhere. He's used them in two leagues before, and they were used throughout the league instead of just at the end. It is so much more consistent for Ash to use his Alola Pokemon for what will inevitably be his last battle in the region. Plus, they're likely gonna have him use three Alola Pokemon at least (Pika, Lycanroc, and obviously Torracat), so it makes less sense for him to bring back half a team of reserves. Regarding his sixth, Poipole is probably the best choice, narratively speaking. He wouldn't catch Guzzlord, the most powerful and destructive UB, and then out of nowhere decide to use it. Rotom would make sense, actually, but I don't think Ash has ever used a second electric Pokemon. Sure, Poipole hasn't had a serious battle, and isn't very good at it, but the Guzzlord episode could certainly change that. It's better to re-introduce Poipole this way than just having Ash somehow open a wormhole because he wants to get Poipole. Plus, Ash has a strong bond with it, why wouldn't he battle with Pokemon he has that kind of connection with?
As for Kukui pulling four Pokemon out of nowhere, remember the writers just gave Alain four other Pokemon solely for the purpose of battling Ash in the finals. Kukui is clearly much more traveled than Alain, and very likely has several more Pokemon somewhere. "Full battle" is in the title. Ash and Kukui are going to be using six Pokemon each. There's no way they won't do that.