While I feel this thread is pointless, since I can't really see the purpose.
I'll respond with:
The writers have been given extra options. I can't help but think from the writer's perspective, that allowing Ash more Pokemon allows the writers a lot more creative storylines. So if one writer wants Palpitoad to be used instead of someone else, he has that opportunity.
Or if someone wants to use Roggenrola, he can use the Pokemon.
Thus it comes down to, I think the current episode's team depends largely on the writer. Now it can be assumed that Ash does trade out the starters. And while that would make sense, the only one we can be sure of is Tepig, but that's only because we've seen Snivy and Oshawott recently than Tepig. However the episodes where Ash has used Tranquill and either Swadloon/Roggenrola at the same time, it can be said it was Scraggy who was sent. So we cannot be sure of who was switched out and who wasn't.
I mean did you notice the Zorua movie, where only 5 of Ash's Pokemon appeared. I get the feeling in those instances its because the writers want to keep who was rotated out a secret.
So again, I don't think we'll ever be sure who is rotated out. But so far of the Unova Pokemon, I'd say Oshawott, and Snivy haven't been rotated out. Tepig/Scraggy are unknown but one of them was rotated out, unless Ash switched out Swadloon for Tranquill after Swadloon made the costumes. However one of them was trading out in the Archeops episode because both Tranquill and Roggenrola were out of their ball at the same time.
But even ignoring all that. I still think it's possible that if Ash sends out six Pokemon it will be: Pikachu, Oshawott, Snivy, Tepig, and Scraggy, and some other random Unova Pokemon Ash has.
But then again the last time that happened was when Ash only had Swadloon as a reserve, and it was easier to switch out Swadloon and Tranquill because Tranquill is a flying Pokemon, and would have a different group dynamic. Such as someone said if Tranquill was in the Palpitoad capture episode, it would've probably avoided the poison spores or gusted them away. But that was also the last time we saw Ash with six Pokemon, so with Palpitoad and Roggenrola, it might be more difficult to show six Pokemon because I don't think the writers want to switch out the same old Pokemon slot for a different reserve.
He's not going to rotate Pikachu anytime soon.
I do hope though at some point, Ash is allowed to use six Unova Pokemon in one full battle that doesn't include Pikachu, but I have a feeling that won't happen.