Alright, also, Tau and Imperium team up quite a lot, generally to deal with universal threats, such as the Tyranids.

[img]http://images4.wikia.nocookie.net/__cb20130503054550/warhammer40k/images/5/5c/Ciaphas_Cain_and_Tau.JPG[/img]

Also, some Inquisitors hire Kroot as mercenaries when performing missions n Jungle worlds. The Imperium are more lax on Xeno's than they let on.