How could I augment my Tau army with Necrons?
While Tau finally got some flyers of their own, the Necron flyers are still cheaper and better.  I would suggest using two squads of warriors in Nightscythes.  You should also run an Overlord.  He won't do much for you, but he unlocks a Royal Court.  Take one of those with a few Storm Crypteks.  Give the crypteks Voltaic staffs to shred hull points off vehicles.  Round it all out with a Doomscythe or an Annihilation Barge.

Alternatively instead of an overlord take a Destroyer Lord with a warscythe and have him lead a unit of wraiths.  Very annoying to deal with.

This will give your Tau some mobile scoring units and a little more durability.