An idea comes to mind. What if, instead of just genestealers, you had a bunch of different aliens, all partially lobotomized and fitted with control implants, as the retinue for an inquisitor from the ordo xenos? He/she could be one of the more radical inquisitors whose philosophy/ methods are viewed with some uncertainty by their peers. The inquisitor could believe that xenos should be made to serve, and can be turned into weapons against those of their own kind who can't recognise the natural superiority of humanity and, therefore, the obvious rightness of our rule. You could have a tau as a gun servitor (I'm imagining a lot of plasma, maybe a tau carbine fixed to his shoulder like a predators plasma caster, and an imperial pattern plasma gun/cannon in place of an arm), an Ork with a variety of close combat fittings (a drill, a buzzsaw, and an Ork pattern power claw), plus a genestealer, and maybe either a vespid or a Kroot (the vespid wings could function like the scavvy wings mutation). Perhaps even an Eldar warlock functioning as a sanctioned psyker. Which would leave you with the human inquisitor as your boss.