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So got my hands on the new Necron Codex and some Wraiths and Scarabs (still working on the spiders).
Whilst I grow my Necron army i will be allying them with my Tau. Any opinions on the below list? If its works out in test matches I may well take it to a tournament.
I think it covers all bases at the moment to be a good Tac list (other than psychic powers obviously) but im sure there are improvements that could be made. Any suggestions?

Tau Commander w/ 2x Marker Drones, C&C node, PEN Chip, MSSS, Iridium Armor, NWSJ, VRT, Drone Controller - 199

3x Crisis Suits w/ 6x Missile Pods, 3x Target Lock, 4x Marker Drones - 219 (Joins the commander)
Riptide 1 - HBC, EWO, Velocity Tracker, Fusion Blaster - 205
Riptide 2 - HBC, EWO, Velocity Tracker, Fusion Blaster - 205

6x Fire Warriors - 54
6x Fire Warriors - 54

Tetra w/ Sensor Spines - 40
Tetra w/ Sensor Spines - 40
Y'Varah w/ EWO, Stims - 270

Canoptek Harvest:
4 Wraiths /w Whipcoils 172
3 Scarab Swarms 60
1 Spider 50

Canoptek Harvest:
4 Wraiths /w Whipcoils 172
3 Scarab Swarms 60
1 Spider 50

Total - 1850

Seems like a good list.

A few things though, have you considered a Fire Base Support Cadre?
You could potentially fit 2 of them in with the Necron Component.

Also why only 4 Wraiths per squad? Why not go six each and drop an appropriate amount of Tau to make it fit. Maybe a Riptide?

I'd also by very tempted to include a big mob of Flayed Ones, I understand they are quite good in the new codex and can be taken as a formation of just 1 squad. Run them up behind the wraiths to charge anything that doesn't die.

Also more for fluff purposes than gameplay, but I feel like it would be more fun to include some form of Necron HQ to represent the leader of that faction. What are the HQ formations from the Decurion detachment?

It's also probably worth pointing out that the the only ObSec units in this list are the 2 fire warriors squads.

I did think about the support cadre but i think it will be less effective than the Buffmander and Missile Crisis. They ignore cover, provide markerlights and already have tank hunter (or monster) and twin linked. It would have benefited the Riptides but then they would have any markerlights. + the Yvarah without the Y'varah very little anti tank.

Your right about the Fire Warriors, 2 obsec squads and a re-roll for the warlord trait is almost not with it. I could drop the 2 FW squads and go unbound .If I also drop a tetra or some wargear i could also afford a basic Necron Barge lord or Destroyer lord. Or keep the tetra and just take 2 more wraiths,

I got some deathmarks, warriors an tesla immortals from a friend but im not sure where they fit in this list. Flayed ones would be awesome eventually. Them and wraiths could make a good CC list.