All i can say, Guard and Chaos Daemons......
This is what seems to throw people off.
Remember, the Imperial Guard is not one homogenous entity. Codex: Imperial Guard is used to represent a vast number of different types of units. High end PDF like the Ultramar PDF. Drudging siege specialists like the Death Korps of Krieg. They could be the fanatical troops of a shrine world. Jungle fighters like the Catachans.
So yeah, if you have Tau allies, it's possible they are humans subjected by the Tau Empire who have been brainwashed like everyone else in the Tau Empire and now fight for the Greater Good.
If they're allied with the forces of Chaos, they are traitor Guard, or heretical PDF converted by Chaos agents.
If they are allied with Sisters or Grey Knights, they are Guard that have been commandeered by Ecclessiarchal or Inquisitorial agents.
Remember folks, if you include the Forgeworld kits, there have been fourteen
different model lines for Guard Regiments since 2nd Edition. Cadian (metal then plastic), Catachan (metal then plastic), Mordian, Tallarn, Valhallan, Steel Legion, Praetorian, Vostroyan, Tanith, Death Korps, Elysian, Traitor (FW). This is before you get into the specialist troops like Stormtroopers, Rough Riders, etc. That alone should tell you something. And people give Space Marines a hard time for getting too many models, heh.
There are billions of them, so the Imperial Guard can be just about anything
you wish them to be. Which is probably why they can ally with just about anybody.