I'm rotating...

1. Misty (Silver Chocobo) - Sentinel, L45.
2. Cait Sith - Medic, L99.
3. Malcolm (Chocobo) - Commando, around L54.
4. Walt (Some robot ravager you get early-ish in the game) - L40.

Misty aside, I've leveled them using whatever I can get my hands on rather than sticking to Potents etc. So I will inevitably get a few more chocobos leveled up to take on the Medic, Commando and Ravager roles in time.