But I do love lively conversation, so let's jump in.
So far I've only run with Harlequin allies, and their own fluff states how they mysteriously choose to show up and aid basically anyone only to vanish again.
Totally agree. Harlequins appear all over the galaxy serving their own ends. Appearing and assisting a Tau expansionist force is in line with fluff as much as Harlequins showing up to assist the Dark Eldar or Eldar. I would even see an argument that could see Harlequins fighting in the same theater for the benefit of the Tyranids or Traitor/none Slannesh Marines (I don't see them ever working with the Necrons).
I figure something along those lines is a good fluff way to explain it. Otherwise it seems unlikely that the Tau would help beings that harvest those they conquer.
The new Necrons are vastly different from their previous incarnation and there hasn't been very much detail regarding their internal politics. As of now, the first-contact event with the Necrons has been the loss of an entire world's population... So working with them would be a complicated scenario indeed. The only other species to have done this to the Tau are the Tyranids (by far the worst with the loss of an entire region of systems) and the Dark Eldar.
Perceived magic or not, Necron technology is based on science and physics, the same as Tau technology.
Being that the Necrons can bend and break the laws of physics as easily as a lightswitch, I'd be forced to disagree. But I understand the crux of what you're getting at- I just don't see the Tau being able to understand Necron tech in the same way that no one understands Eldar tech because of the psycho-relationship a user needs to have with everything. If the Tau were able to reverse engineer Necron tech they'd just 'win'. I'd also argue there's probably a cultural resistance to Necron tech as it seems to involve the vampiric reduction of beings and stellar bodies while the Tau are 'humanists' and environmentalists to a fault.
I'd have to read someone's fluff to get on board with a conversion, in short, but it is possible if unlikely.
When I say I don't 'get' allies it's because I come from an older batch of 40k player. Unlike Star Wars or Star Trek where aliens are working together constantly, 40k's attraction to me is the inability for the factions to work together. The adoption of 'let's stop shooting each other and shoot at that big guy and when he's dead we'll go back to shooting eachother' was never as intriguing as 'we'll shoot through each other and maybe that will kill that third guy'. If that makes sense.
But your own fluff:
makes sense and there's a precedence of Tau mercenaries working further afield in the Eastern Fringes. If you were playing the fluff angle you just wouldn't see Riptides and the higher end Tau technologies- but that's a flavor thing. The concept is sound.To paraphrase my army's fluff greatly my contingent are working hand-in-hand with two Ordo Malleus Inquisitors.
I own a knight and I bring it out occasionally. One of my inquisitors is a very powerful psyker.
Now, personally, this is where I would get off-board. You have two very powerful psykers, they have cultists, you have a radical Ethereal (what is a radical Ethereal?) and a large compliment of Fire Caste. They have access all over the galaxy and can call on a knight which is a daemonic entity- that's a lot. It would already be more of a heresy to be working with aliens let alone being a powerful enough psyker to be able to manifest a daemon on a whim. Remember that the Traitor Marines themselves do not regard the use of aliens as palatable- and these guys eat babies... Your army would be the greatest series of heresies that would dwarf any other radical Inquisitor's accomplishments in the history of the Imperium (read Dan Abnetts stuff).
But this seems to be a style of play and a style of fluff- and I respect it, but I personally just think that's too much boom.
But that's not an attack- as stated in the beginning, I just like bing more hardline on 40k fluff.
I'll read your stuff.