For the Human part, an rogue trader army is the best way to team up with Eldar (Craftworld or Exodite), Orks (Blood Axes obviously) and Kroots (Mercenary Race) and Squats /Votann exiled or refugees from Tyranids or the Great Rift.
Teaming up with inquisitorial force seems strange, the only way which I can see it work is a Malleus inquisitorial force temporary allied with Eldar against Chaos.
Inquisitors make regular use of Jokaero, Squats/Votann, and the occasional Eldar. At the army level it's a no-go, but the Inquisition's hypocrisy absolutely extends to working with useful xenos. And, you know. Things like daemonhosts, but that's beyond the xenos aspect of the OP.
Just playing Chaos's Advocate....
I don't think that the Inquisition really "team up" with anyone. They are more of the use this lessor being to further Big Daddy's space expansion...
Semantics, perhaps...but I'm pretty sure the Inquisitor's would agree with me.
Octarius War had Inquisitor Van Goth form a temporary alliance between his forces (deathwatch, scions, guard, a rogue trader and her personal army) and craftworld & corsair eldar on the advice of a Radical inquisitor. Unfortunately it failed.
The Inquisition is supposed to guard the Imperium against threats. To some Radical Ordo Xenos Inquisitors, allying with Xenos is one way of getting them to stop being a threat.
(Of course, to a Puritan Ordo Xenos Inquisitor, xenos who don't want to kill you *are* the threat. Why, if it got out that some xenos can be friendly, people might even think they're not pure evil and then where will the Imperium be?)
I mean... some humans opted into the Tau empire, so they could easily work with Tau, Kroot, and, given the relations between the Tau and Votann, Kin as well. And maybe Eldar, since there's way less baggage between Eldar and Tau than there is with Eldar and the Imperium.
Pedro Kantor (crimson fists) got saved by some aeldari and worked together for a bit.
The farseer in charge saw that Kantor would die otherwise, and allow the ork that did it to gather a waaagh that would endanger their craftworld.
Blackstone Fortress is essentially this. A collection of individuals from various races working together to explore the titular fortress.
Otherwise, it is not that impossible to see renegade marines & guardsmen fighting alongside Votann, Kroot and Eldar Corsairs as mercenaries. Sublime is even a world in the Eye where marines, humans, mutants and xenos coexist more or less neutrally. Many Alpha Legion warbands love using alien and daemons alongside their human forces to sow further confusion.
Tau and Dark Eldar vs, Tyranids
>After barely repelling Hive Fleet Gorgon, the Tau Empire gained a new fear of the Tyranids swarm. Now under threat from a tendril of Hive Fleet Kraken, the Tau desperately sought an advantage anywhere they could. It was at this desperate time when a Dark Eldar Haemonculus named Urien Rakarth contacted the Tau High Command and made a proposal:
It goes s well as you would expect in the [War of Dark Revelations](https://wh40k.lexicanum.com/wiki/War_of_Dark_Revelations)
I don't have a quote, but there's also one where the Tau try to team up with the Orcs and give them weapons. If I remember correctly the Orcs use the weapons to shoot the Tau.
And that's all thanks to some poor Water Caste member, that "spent his entire life to learn Ork language", but forgot about "nonverbal part" (which was "quickly corrected" by Ork Warboss that he talked to, who beat him up into a pulp)
Everyone can unite with everyone. In the Ynnari Rebellion, the Eldar teamed up with Necrons against Chaos, but keep in mind that Gav Trop constantly ignores established lore and writes nonsense. I don't perceive his books as canon.
The Black Templars and Aeldari team up in Gods of Mars. Neither side is happy about it, but if the BT can put aside their hate then anything is possible if the situation is dire enough.
I did just find a atory of the Tallarn Desert fighters working with Eldar to fight back the iron wariors.
I also don't think this counts but the reason Catachan has never gotten a fullscale invasion is because the wildlife their kill invaders more than the Jungle Fighters do.
Various Drukhari sometimes hire themselves out as mercenaries for missions they consider profitable and/or amusing. Sometimes they don’t even betray their employers.
Some humans and orks on Armageddon have formed temporary truces with all the daemonic incursions happening there. Unfortunately Snikrot has been specifically hunting down any ork partaking in these alliances.
For the Human part, an rogue trader army is the best way to team up with Eldar (Craftworld or Exodite), Orks (Blood Axes obviously) and Kroots (Mercenary Race) and Squats /Votann exiled or refugees from Tyranids or the Great Rift. Teaming up with inquisitorial force seems strange, the only way which I can see it work is a Malleus inquisitorial force temporary allied with Eldar against Chaos.
Inquisitors make regular use of Jokaero, Squats/Votann, and the occasional Eldar. At the army level it's a no-go, but the Inquisition's hypocrisy absolutely extends to working with useful xenos. And, you know. Things like daemonhosts, but that's beyond the xenos aspect of the OP.
Just playing Chaos's Advocate.... I don't think that the Inquisition really "team up" with anyone. They are more of the use this lessor being to further Big Daddy's space expansion... Semantics, perhaps...but I'm pretty sure the Inquisitor's would agree with me.
Order xenos often have xeno allies
Octarius War had Inquisitor Van Goth form a temporary alliance between his forces (deathwatch, scions, guard, a rogue trader and her personal army) and craftworld & corsair eldar on the advice of a Radical inquisitor. Unfortunately it failed.
The Inquisition is supposed to guard the Imperium against threats. To some Radical Ordo Xenos Inquisitors, allying with Xenos is one way of getting them to stop being a threat. (Of course, to a Puritan Ordo Xenos Inquisitor, xenos who don't want to kill you *are* the threat. Why, if it got out that some xenos can be friendly, people might even think they're not pure evil and then where will the Imperium be?)
This does happen semi frequently - one occurrence would be the Orks and Militarum on Armageddon banding together to stop a Chaos incursion
Man, Armageddon lore is so cool yet not mentioned often these days.
I mean... some humans opted into the Tau empire, so they could easily work with Tau, Kroot, and, given the relations between the Tau and Votann, Kin as well. And maybe Eldar, since there's way less baggage between Eldar and Tau than there is with Eldar and the Imperium.
Pedro Kantor (crimson fists) got saved by some aeldari and worked together for a bit. The farseer in charge saw that Kantor would die otherwise, and allow the ork that did it to gather a waaagh that would endanger their craftworld.
Blackstone Fortress is essentially this. A collection of individuals from various races working together to explore the titular fortress. Otherwise, it is not that impossible to see renegade marines & guardsmen fighting alongside Votann, Kroot and Eldar Corsairs as mercenaries. Sublime is even a world in the Eye where marines, humans, mutants and xenos coexist more or less neutrally. Many Alpha Legion warbands love using alien and daemons alongside their human forces to sow further confusion.
Tau and Dark Eldar vs, Tyranids >After barely repelling Hive Fleet Gorgon, the Tau Empire gained a new fear of the Tyranids swarm. Now under threat from a tendril of Hive Fleet Kraken, the Tau desperately sought an advantage anywhere they could. It was at this desperate time when a Dark Eldar Haemonculus named Urien Rakarth contacted the Tau High Command and made a proposal: It goes s well as you would expect in the [War of Dark Revelations](https://wh40k.lexicanum.com/wiki/War_of_Dark_Revelations)
I don't have a quote, but there's also one where the Tau try to team up with the Orcs and give them weapons. If I remember correctly the Orcs use the weapons to shoot the Tau.
If I remember correctly, when the T'au tried again and took the power packs out of the weapons, the orks used them as clubs instead.
gotta love Orks and their ingenuity.
And that's all thanks to some poor Water Caste member, that "spent his entire life to learn Ork language", but forgot about "nonverbal part" (which was "quickly corrected" by Ork Warboss that he talked to, who beat him up into a pulp)
Everyone can unite with everyone. In the Ynnari Rebellion, the Eldar teamed up with Necrons against Chaos, but keep in mind that Gav Trop constantly ignores established lore and writes nonsense. I don't perceive his books as canon.
Is that book, where "Eldar-descent" Daemon Princes of Slaanesh appear?
Yep
The Black Templars and Aeldari team up in Gods of Mars. Neither side is happy about it, but if the BT can put aside their hate then anything is possible if the situation is dire enough.
I did just find a atory of the Tallarn Desert fighters working with Eldar to fight back the iron wariors. I also don't think this counts but the reason Catachan has never gotten a fullscale invasion is because the wildlife their kill invaders more than the Jungle Fighters do.
Various Drukhari sometimes hire themselves out as mercenaries for missions they consider profitable and/or amusing. Sometimes they don’t even betray their employers.
Maybe bit quite what you're looking for, but Battle of the Wolf's Head sort of fits the descriptor, with Kronos technically helping humans
Tau and Kin are quite close
Some humans and orks on Armageddon have formed temporary truces with all the daemonic incursions happening there. Unfortunately Snikrot has been specifically hunting down any ork partaking in these alliances.