They are considered to be part of the Imperium of Man, so therefore that is where they are located cart blanche for ally purposes. Basically GW is lazy as fuck when updating the Ally Chart and made all of the Imperial factions (Space Marines of many flavor, Imperial Guard, Inquisition, Imperial Knights, Sisters of battle, Ad Mech + Friends) are all added together.
Pending Fluff arguments IG and Ad Mech could be used as Allies of Convenience with certain outside forces - such as CSM - if you talked it over with your friends.
Originally Posted by neilbatte
if you squint the Sigmar stuff doesn't all look like the love children from a Necron and Blood Angel orgy.