One of the Missions I ran had the Kill-Team unexpectedly aiding the Tau.
The Tau had a scientific research vessel orbiting one of the few agri-worlds of the Reach. They were studying various flora and fauna of the Reach, cataloging hardy strains for transplant to Tau-controlled worlds. Among the many cryo-stored samples were some of the Tyranid spores that accelerate plant growth…the idea being that hardy food bearing plants could be genetically spliced with the spores to produce larger quantities of food in a shorter period of time. There was an accident aboard the science vessel and it crashed to the planet, sinking deep into one of its seas. In the crash, many samples were released through container damage, including the Tyranid spores. In the immediate area of the crash site, kelp and krill were altered by the spores; the kelp grew prolifically and began to cover large swaths of the sea's surface, and the krill began to multiply to frighteningly massive numbers. Whale-like creatures that fed on the krill migrated to this area of the sea and began to gorge themselves on the seemingly unending supply of krill food, and were in turn mutated.
The Tau sent emissaries to the planet to smooth over relations with the planetary regent, and were soon on their way to incorporating the planet into their empire, when the Kill-Team arrived in response to Imperial Navy assets alerting them to increased Tau fleet operations in the area. The Tau were busy trying to recover and salvage what they could from their crashed science vessel, and treating with the planetary regent on equal footing. The Kill-Team's arrival to the planetary mansion was a surprise for them. The drive up the main approach was bounded on both sides by an immense orchard, and the Kill-Team could see strange "things" flitting through the branches and trunks, but no one got a good enough look to identify them. The regent met the KT in the back lawns, dressed in a bathrobe, shotgun in hand and shooting *****, while a Tau Commander in a Crisis Suit stood by and watched. As the KT were preparing to "take care of" the Tau Commander, swarms of Gun Drones arrived from the orchards…and the Kill-Team stood down until they could figure out what exactly was going on.
The Tau really had no interest in the planet, other than it's flora and fauna, and were willing to leave if the Imperium would let them gather what they could from the crash. The Kill-Team opted to help the Tau in their recovery operations, expediting their exit of the system.
On the open seas, aboard a Tau research vessel, the KT acted as overseers to the recovery operation. They were aware they were being scrutinized by Tau Water Caste researchers…and closely watched by hidden Stealth Suits. As the last of the salvage was being brought to the surface, one of the mutated "whales" attacked the floating research ship. I essentially ran the encounter as if they were combating a water-dwelling void drake. Massive bio-lightning attacks that ripped hull plating from the ship, fried Stealth Suits, and created a dangerous environment in which the KT had to fight for their lives.
Huge bio-lightning squelching Tyranid whales hoisting themselves aboard the science ship, attempting to capsize it and devour its occupants. The deep, deep sea surrounding you as far as the eye can see, filled with a morass of horribly tangled kelp (and billions upon billions of biting krill). A sinking ship occupied by opportunistic backstabbing Tau Stealth Suits. It was like Titanic meets Moby **** meets Alien. Water and bio-lightning spell disaster for someone unfortunate enough to be caught in the mix. Good times. I think I stripped two Fate Points with that Mission.