So, if I played my Ultramarine Honor Guard counts as Grey Knights as an Allied Detachment for my Necrosn, would the universe explode?

I'm really like playing games with my new Necrons.  I've been doing fairly well with them, and I don't feel like I'm being punished as much for my codex choice.

My list has generally been something like this:
2x Scythe Overlords in Command Barges
2x 5 man warrior units with cryptek in Ghost Arks
2x 5 man warrior units with cryptek in Night Scythes
1x 15 man warrior unit with res-orb Lord
2x 4 base scarab units
2x Annihilation Barges
1x Doom Scythe

I would like to work some Wraiths into the list, but I'm holding back on spending any more gaming money for a while(Went kind of deep into the Relic Knights Kickstarter).  Until then I'd like to add a little more kick to the list.  I want access to offensive psychic powers too.  Grey Knights are the obvious choice, but my grey knights are modeled as Ultramarines Honor Guard.  That ought to give the occasional fluff bunny a fit.

I wish the state of the game was not such that you need to ally GK or SW or Guard to be competitive.  But we seem to be transitioning towards Death Star 40K.  So you need to able to field your own Death Star, or be able to deal with one; preferably both.  You can't always do that with one codex, and if you are going to ally, you need to ally the best.

Hopefully the current Death Star trend will implode on itself.  I'm hoping that it's partially due to Nova not quite getting the competitive format right for 6th.  Nova table quarters doubled down on punishing vehicles for being non-denial.  The competitive format needs to either make more use of making heavies and fast attack scoring(maybe even elites too), or allowing vehicles to act as denial in your own deployment zone.