I do notice that they conveniently kept Kukui out of any situations where him revealing his Pokemon and battling would have been logical up until the Kommo-o lair. And from the second episode we knew Kukui had Pokemon just he didn't get to show them because Tapu Koko interrupted his intended battle.
That said I would like it if his Pokemon got some spotlight. They could add to his eccentric teachings. (I can understand him hiding Incineroar for the time being though).
Concerning the other students, I guess it's just a case of current potential. I've noticed Ash and Kaiwe's teams getting used more often in Season Two besides skill boost instances, though I think it's not just a case of power level, but the number of abilities and quirks they can put into anything right now, while the others are still limited to a couple basic type based moves so are stuck in 'Aquaman' roles where they can only help in their element. Check out last episode, a large helping of Ash and Kaiwe's Pokemon got to help out against TR in some amusing way, meanwhile the girls were stuck in the background helpless and while Sophocles' Togedemaru helped, it was largely because A Meowth randomly had an electric attack. The girls and Sophocles need some divergent skill boosts.
Kaiwe makes sense being another battler in this episode, while Sophocles is probably the one who next needs to get experience if Ula Ula is coming up. Charjabug has potential to expand after all, it can dig and use String Shot already and in a Fire dominant battle royal it's already trying outside it's element.