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.