It should be possible, instead of specifying a specific pet (in a team), to specify a slow as a levelling-slot.
Another place in PBT you specify a levelling-queue of pets.
Then, when you use a team where one (or more) slot is assigned as Level-slot, PBT automatically swaps in the first pet in the level-queue.
Once the pet reach 25, it's removed from the level-queue and the next pet takes the place in the level-slot.
This way, you don't need to change your teams for each pet you want to level, as long as you remember to keep pets in the queue..