Than I was thinking Marines who had been declared traitors ran into a severe warp storm and were left crashed and stranded on a Tau planet. These Marines were witness to some horrific act at the hands of the Imperium, which caused them to turn renegade in the first place, so they no longer have any loyalty to the Emperor. If they're low on supplies and surrounded by Fire Warriors, would they really choose to die for something they no longer believe in?
1. Astartes who are declared Traitors usually fall in to two categories: go totally rogue (usually Chaos SM after that), or try and redeem themselves.
2. Horrific act? Astartes are bred for horrific acts. It's what they do. What's the worst that can happen? Genocide? Ask the Ultramarines and Inquisitor Kryptmann about that. It happens regularly and Astartes are usually involved.
You've got to understand the psychology of Astartes: they are psycho-indoctrinated in the beginning to be unbelievably loyal and steadfast...loyal unto death...literally. Their lives are spent in constant ritual to reinforce this.
To lose their faith in the emperor, they would have to experience something horrific, usually directed at something they hold more dear than humanity...most likely themselves or their Chapter. Even inquisitor go against Chapters of Space Marines exceedingly rarely because they are such a valuable tool, and only go after them when things are already deeply wrong: mutation, no longer tithing geneseed/ hubris, heresy, etc. (if anything they are ripe for the predations of Chaos, feeding on their greed, hubris, pride or rage.)
"The Greater Good" is a fine idea, but the Astartes already believe in the greater good of wiping the Xenos out...period. "For the greater good OF HUMANITY"
Surrounded by Firewarriors, out of ammo, Astartes will charge and throw rocks before surrendering. They don't know the word "surrender". The purpose of "surrender" is to save their lives, and their lives are already bound by psycho-indoctrination, oath and creed to die in battle in service to the emperor.
Sorry man, while the Tau would easily be the lesser of two evils in a firefight against say Tyranids, or Necrons, or Chaos...I can honestly say they would NEVER...
EVER...simply join the xenos and their crusade for the "greater good". Not a snowball's chance in hell.
As an aside, I like the idea, even if it would never work, it's imaginative and I dig it. It's just that it is likely to work as a Tyranid/ Necron Hybrid or Eldar joining the Orcs. Please don't take my
opinion as an attack or anything, you've done great in coming up with something original. Cool?