That works as well. I apologies, I misread the original combo there when writing that up. But if it works, it works.
So we have a band of special forces, space based siege breakers, who abandoned the Emperor's Light by aligning with the very xenos they meant to purge, so they could endure and continue their mission.
The Iron Bloom were all too willing to provide aid to the Guard during their time of isolation, feeding their starved hive city, repairing vital systems, and after a long period of time, the humans actually found they preferred the xeno crafted weaponry.
Their orbital strike pods, the key to their siege breaker abilities, is just as much a mount as it is a vehicle, driven by a bestial AI with an appearance to match.
Their effectiveness is without question, but the crime of their use is unforgivable to the AdMech, who seeks to destroy the heretical regiment.