Examples I could think of off the top of my head:
- I think the actual concept of Clan "contract warfare" (as coined by the Horses) was the Horse/Coyote collaboration against the Smythe-Jewels, so I thought I'd include this first!
- The Harvest Trials, especially knowing that hundreds of warriors from other Clans intentionally offered themselves up for this in order to get a shot at the Inner Sphere. It's actually how Vlad and Malavai ended up coming to their arrangement.
- The Golden Ordun was originally formed by the Horses submitting their Fire Horse Galaxy to Malvina Hazen and merging it with her own Omega Galaxy, and she later straight up stole the Galaxy later on in a Trial of Possession. The Horses' Gamma Galaxy I know was pretty involved with Malvina as well, and likely other Galaxies that I'm just not remembering.
- The Burrocks contracting themselves out to the Star Adders and Nova Cats to transport their exploration and scientific teams and share in any profits via futures trading.
- The Burrocks, and later the Adders, sharing in the garrisoning of the Tanite Worlds with the Cobras in exchange for a cut of the profits.
- The Steel Vipers garrisoning worlds for the Ghost Bears would seem to qualify, I'd think.
- The Sharks offering up their Potemkins and Beta Galaxy to the Nova Cats for the evacuation and defense of Barcella during their Abjuration in exchange for the Cats' assets there. Didn't the Ravens help them move as well?
- Maybe the Adder naval forces that were placed under Terrance Moffat's command when the Viper/Adder naval force ambushed that big Raven convoy, since the operation was specifically under Viper command?
- Maybe the Exiled Wolves (and even more of a stretch, Wolf's Dragoons) when they were under Alaric's command for the home stretch of the invasion of Terra?
- A very peripheral and quasi-canon example: in MechAssault: Phantom War, the Hell's Horses had formed an alliance with the primary antagonist of the game, a Lyran militia commander.