I'd like to say outright that I haven't read any of the new supplement books (I don't have an Apple product and none of them seem to have hard copy available yet) so it is possible this question is answered in them. But I'm confused why people are saying things like "Black Legion lets you take an extra hellturkey" etc. I was under the impression that a supplement book supplied some optional special rules for a particular subdivision of a force, but Black Legion are still CSM and Farsight is still Tau and even if every craftworld gets their own book they'll still all be Eldar.
So if you cannot ally with your own army why on earth would playing a supplement list let you dodge this? After all you are allowed to play Black Legion without having the supplement and those Black Legion cannot ally with, say, Alpha Legion. I guess the way I am reading the rules is that an ally has to require a different army book, and supplements fail that test.