Easily.
IG, like other humans, constantly turn traitor for a wide variety of reasons. It's easy to imagine that an
IG unit could decide that fighting for money is a lot better than being expended as suicide troops in the name of the Emperor, with or without chaos corruption being involved. And it's equally easy to imagine them finding plenty of buyers for their services, especially in areas on the edge of Imperial control where even incidents that aren't forgotten entirely take ages to be dealt with. After all, the thought that centuries later your descendents might finally be punished for treason is pretty irrelevant when you've got a war to win right now.
Loyalist, Emperor-loving Tau.
No. The Tau have a fundamental belief in their own destiny to rule the universe, with all non-Tau submitting to the Greater Good of Tau rule. I suppose you could make up some kind of excuse with "corrupted Ethereals" or whatever, but it goes directly against everything we know about the Tau (including the entire concept of their design and role in the fiction) and is far likely to fall into the "bad fanfiction" category than become a good story.
Space Marines that are not well, SPEESH MAHREENHS
No. Space marines exist for two reasons: to kill their enemies, and to produce propaganda for the Imperium*. The most you can get is chapters like the Ultramarines or Raptors that use sensible tactics to kill their enemies instead of blindly charging in with chainswords, but in the end they're still space marines. They kill stuff, and they do so at close range where they can be the GLORIOUS WARRIOR HEROES OF THE EMPEROR. Expecting them to be useful in diplomacy is about as sensible as expecting a nuclear missile to make a good diplomat.
*Overpriced elite infantry make no sense in a world of tanks, aircraft and orbital bombardments. The inevitable conclusion is that space marines exist primarily to be inspirational heroes in the propaganda that unites the Imperium, with their actual effectiveness in battle being a distant secondary concern.
Peaceful, noble orcs.
It depends.
If you're doing it seriously, it goes directly against the entire concept of the orks. Like the Imperial Tau, the most likely result is bad fanfiction.
If you're doing it for comedy value, it could work. After all, orks are there to be comic relief, and a story about pacifist orks could be funny if done right.