Bouffalant: Provides the orange buff- your attacks will slow down your enemies
Ludicolo: Provides the purple buff- your attacks deal more damage against Pokémon with low health
Rotom: Upon being defeated, it will go to the enemy’s goal zone- if it reaches it, it will go in to the goal zone, making scoring much, much faster
Drednaw: Upon being defeated, your team gets EXP points and shields
Zapdos: Upon being defeated, the enemy’s goal zones will be opened, eliminating scoring time (you score immediately as you start charging). It also gives your entire team 20 points.
Source: I’ve gotten to the Veteran 2 rank, and I used this article to double check
Hope this helps!