So, I popped the rough draft up on RPG.net, to see if there were any glaring discrepancies/omissions from my statline.
Scarik, RPG.net's resident RT head, helped to shape the statline a bit to make it more in line with the representation of Tau elsewhere. So, as follows, your basic Tau, suited to a Rogue Trader level of play:
Roll 2d10 and add...
For the Greater Good
Tau have an overarching belief in "The Greater Good", a concept that all should be equal, and everyone has their place within the Tau Empire.
This sets them at something of a disadvantage when dealing with others - they suffer a -5 penalty to any Fellowship tests against those who hold another strong religious belief (so, Kroot would listen to them, but an Emperor-Fearing Human would be hard pushed to listen to the heathens).
Being "Space Commies", they are also somewhat less concerned with money than most races - meaning that they cause any group they are part of to suffer a -1 to their Profit Factor.
Tau have a very small Warp presence - so much so, they seem to be absolutely naive about the workings of the Warp. Tau can never become Psykers, of any sort. They also suffer a -10 penalty to any Forbidden Lore skill which relates to the Warp (so, Daemonology, The Warp, etc.) However, they are also far less likely to gain the attention of such beasts - they gain a +10 bonus to attempts to resist possession by a Warp entity, and GMs should ensure that the creature has either a specific plan to target the Tau, or no other choice.
Tau and Insanity and Corruption
Tau gain Insanity points and the ensuing complications as normal - after all, the horrors of war and the strangeness of the Warp affect anyone who thinks about them too long.
Corruption is somewhat different - instead of representing their fall to the Ruinous Powers, it represents their removal from The Greater Good. As they lose some of their "innocence" regarding the universe, Tau begin to waver in their faith in the Tau Empire.
A Tau who gains Corruption points does not roll for mutations though malignancies (where appropriate ) should still be used. As they move away from the Greater Good (and the Ethereal Caste in particular), Tau begin to revert to a more "feral" state, becoming quick to anger, prone to infighting, and somewhat more vicious and sadistic in their violence. The presence of an Ethereal can negate some of these effects - when an Ethereal is present, a Tau does not gain Corruption points, and their current Corruption point total is considered to be half its actual value.
Choose your Caste wisely - this will affect your stats, and also limit your choice of Careers:
Air : +10 Ag, -5T, treat any one Pilot skill as a Trained Basic Skill. Gain a choice of 2 of the Catfall, Leap Up, Rapid Reaction, and Weapons Training (Tau Basic). Talents.
Earth: +5 WP, treat any two "occupational" skill (frex, Tech-Use (Tau), Wrangling etc) as Trained Basic skills. Gain a choice of 2 of the Foresight, Iron Jaw, and * Talents.
Fire: +5 BS, gain Weapons Training (Pistol, Basic, and Heavy Tau Weapons). Gain a choice of 2 of the Quick Draw, Combat Formation, Marksman, and Disarm Talents.
Water: +5 Fel, treat Barter and one of Commerce or Charm as Trained Basic Skills. Gain a choice of 2 of the Air of Authority, Polyglot, Total Recall, and Paranoia Talents.
*Gonna fill that in later...
Ethereals aren't the best PC option, so I'm not touching them with a bargepole.
The Caste system replaces the normal Origin Path, hence giving a few more bonuses than usual. Tau PCs may (with GM approval) modify the existing Origin Path, re-naming and replacing certain options as the player and GM see fit, to represent "renegade" Tau, or those who have somehow avoided the speciation of the Caste system.
Careers to come soon! I plan on modifying the Void Master for the Air Caste Pilot, the Arch-Militant for the Fire Warrior, the Senechal for The Water Caste Negociator, and possibly creating something from scratch for the Earth Caste Engineer (maybe based around the Adept from Dark Heresy, with a few steals from the Tech-Priest/Explorator). After that, maybe a few Alternate Career Ranks for each, to represent specialities and divergences (like Pathfinders and Crisis Suit Pilots for the Fire Caste).
I'll also post up a few new Talents for the Careers...