Skitarii joining the Tau?
Recently after getting back into 40k I've been having some difficulty sticking to one army or another. One of my friends who plays Chaos space marines has been insisting that I just collect whatever and find a way to play them altogether. This would include Skitarii , Guard , space marines and Tau forces. The issue I have with this is for one i'm pretty sure there is nothing in the rules that would let me do this simply because the Tau are not ,last I checked anyway, not battlebrothers with any of those factions. On top of that I was having a really hard time coming up with a reason for the Skitarii and by extension any forces of the mechanicus / hereteks to join the tau. I figure they might be like a space marine that "Joins" the tau more or less just acting as an ally from time to time without really joining the Tau empire as a whole. I suppose given the Tau's tech a heretek might have at least some interest in studying their technology .
Can any of you come up with a reason a Skitarii group would join the Tau? Although I find myself liking the idea it seems downright heretical to me.