I would hesitate to have anyone play an Ethereal since GW has been so tight-lipped about what makes them tick and their ultimate motives (they've definitely been playing up their sinister elements in recent fluff). Still, a low ranking Ethereal as the group's negotiator is doable.
For the Vespid, maybe make them similar to the Drone Operator in that you play the Strain leader and start with two comrades, with the ability to buy up to 2 more. Technically speaking, Vespid are incapable of communication with the Tau unless wearing the Communication Helm (again, implied to induce mind control). Could be offset by "If this helmet is destroyed or removed, all communications are at -50" rule.
For Gue'vesa, I'd make it similar to the standard Regimental creation system, but the squad gets Common Lore (Tau Empire), Language (Tau) and +20 to starting Logistics. All human enemies that engage them have the Hatred (Gue'vesa) talent, and the squad suffers a penalty to hit enemy Priests (since they are allowed to worship the God Emperor within Tau space).
I remember reading a book where a human tried to pilot a Battlesuit in desperation, but he was cooked alive inside the cockpit within seconds of the hatch closing. Not sure if this was a defense mechanism or the human body can't handle the battlesuit, so maybe lock Gue'vesa out of the use of battlesuits and instead open up Tau Vehicles and Imperial ones (fluff is pretty clear that human auxiliaries have industrial capacity to make their own lasguns, so the ability to manufacture or restore some Imperial vehicles wouldn't be lost on them).
Only thing I can see missing is a dedicated Operator for Devilfish, Piranha or Hammerhead vehicles. Unless the Drone Operator was meant to cover that? Maybe make a Sniper Drone Advanced Class for Operator instead?