Honestly? I'll knock some obvious ones off right away. No Necrons or Tyranids, for obvious reasons most likely.

Beyond that? Dark Eldar would not have any real reason to aid a human, so they are out unless a really, really good case can be made for em. Daemons and the like wouldn't be really welcome, not in the retinue of an Ordo Malleus inquisitor that gets as much attention as he does. Beyond that, I try and take things on a case by case basis.