Not yet... It is a clever strategy. Under the current Necron lore, the goals of the Tyranids and Necrons are fully compatible just as you laid out. It would have been more interesting under the old Necron lore, where their goals would have been directly opposed to one another.
Meh, to be honest, the tau are a fairly interesting race in so much that they appear to be one of the few civilizations to at least sorta have their shit together. The orruks are too feral and uncivilized to really "have their shit together" and the humans and elder empires are slowly falling apart and the necrons aren't exactly a civilization anymore with 99% of them being mindless automatons. The tau are the only civilization to actually civilize and not just be constantly trying to stave off impending doom. Though I find it peculiar that such an order in release of books has come around. An yeah, chaos being the big bad threat isn't fun... it's too much of an inevitable force of nature. To be honest that's the one thing I don't like about warhammer lore in general. Every single scary alien or monstrous race pales in comparison to the ruinious powers and the threat they pose and ultimatly no-one can actually beat em nor is there any option for cooperation or faustian pacts as chaos is hell-bend on doing horrible stuffs to everything... Also, what's wrong with the blood angels? They're just space marines, they don't seem exceptionally poncy. What was the old necron lore that they would have opposed the tyranids?
Under the old lore the C'tan were in charge of the Necrons (there was no uprising against them and they weren't reduced to shards). The C'tan were originally gods that feed off of stars, but soon learned to feed off of other living things (as well as their own kin... which is why only 4 survived). They ravaged the universe and much of the life was destroyed. They went into stasis to give the universe time to re-seed life. So in essence, the C'tan (and as a result the Necrons) would have been competing with the Tyranids for the very same thing.
Ah, so the lore simply hadn't progressed as much as it has currently but the basics remain the same... Meh, two great hungering monster factions isn't of much interest. Though at least the C'tan do need to keep something alive if they want to survive themselfs so they differ in that I suppose. Also, that has the same issue that the current Death faction in AoS has due to nagash being it's supreme ruler. It's too unified and organised and thus would wreck pretty much everything of comparable power. Which isn't very interesting... At least with more than 4 rulers there's a significant option for infighting and conflict to keep it in check.
The beginning of the origin story remains the same They are hardly a unified faction. The C'tan are constantly at odds with one another. It was the C'tan the Deceiver who first convinced his brethren that the greatest sustenance could be attained through the consumption of their own kind. C'tan fought and consumed one another until only four remained [Nightbringer, Deceiver, Void Dragon & Outsider]. I would have preferred ifGW had expanded upon the Void Dragon and the Outsider and brought them into the fold. Maybe even introduce a few more lost C'tan that were originally thought to have been destroyed. With a collection of C'tan led armies, you could have a whole slew of infighting and civil war. Either way, by their very nature, the C'tan are very different from a singular power such as Nagash.
I suppose.. still godlike beings tend to be a tad problematic to write stories around as main antagonists/protagonists.
Treat the C'tan as the driving force behind the Necrons. Your antagonists/protagonists can easily be those who serve under them. Besides, everything in 40K is over-powered, a couple of star gods will be right at home.
Well you can all stop complaining about a lack of a codex, as one's coming out before March ends. https://www.warhammer-community.com/2018/03/11/forgebane-pre-order-next-week/
if the followers are mindless automatons they don't make for particularly good characters. Similarly Tyranids aren't the most exciting to read about either.. at most you're going to read about a party hunting tyranids or a defense against a tyranid invasion. But not about the Tyranids themselves.
The possibilities are nearly limitless. A mindless automaton does not make for an intriguing character, but what about one that is slowly remembering the life he/she/it had? Besides, there is a very large difference between the over-arching story telling that takes place in a codex versus the in depth character exploration of a novel. The codex fluff sets the stage for you to continue your own story forth on the table top.
Speaking of Codices: I heard the Tau Codex is now available! Seems I will have to get it and take a look... But I am still heavily blessed by Nurgle (the third week! I haven't been that sick in 20 years or so!) So not happening much this week I guess.