I'd assume the Stim and buff would not stack in your scenario because leveling pets is already pretty fast. I think the Profession most affected would be Chef. They create specific supplements for a specific group of pets, so the second you're able to get an entertainer buff to cover pet XP generically, there is no reason for a chef to make supplements anymore - so then when a CH can't find an entertainer, they're also not going to be able to find supplements. You kind of end up messing up a quite a few things pursuing this.
This sounds like an issue with balancing the buffs more than the concept of an Ent buff for pets. We already have food and ent buffs for players that people can choose one or the other or compliment them together. Seems like this is a good idea in concept, even if the devil might exist in the details.
