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So I recently purchased a tau army, my current set up is as follows any advise on what else i need would be helpful. i am planning on purchasing the following already 1 more hammerhead 1 more piranha 1 farsight 1 sniper team 2 devilfish and possibly aun va, this isnt to all field at
once but i like to be flexible in my set up for troops. I will be fighting on a regular basis a necron army, tyranid, and dark elder/harlequen. we typically play 2k games but can range to 4k points on occasion when the chaos guy shows up who has over a 12k points worth of chaos units.

30 fire warriors
22 breachers
6 stealth suits
9 crisis
3 crisis body guards
2 hammerheads - interchangeable weapons 1 with long strike
1 skyray gunship
1 comander
1 coldstar battle suit
2 piranha
1 drone port
2 gunrings
2 shield walls
2 stormsurge
1 ghostkeel
1 cadre fireblade
1 darkstrider
1 shadowsun
1 broadside
1 riptide
2 ethereal on disk
2 grav drones
16 shield drones
2 shielded missle drones
2 stealth drones
8 gun drones
21 marker drones
and 3 missle pods

This is a nice list of your army but do you have any sense of how you'll organize it?
You have a lot of units available that will let you run a variety of formations. Against necrons you want a lot of AP 3+ so you can bring them down to 1 save. or str 8-10 so you can make RP harder.
Nids - you want skyfire and volume fire.
Deldar - Ignore cover

You have enough for The Hunter Contingent -
Command
1 Commander
=1 comander
0-1 Fireblade
=1 cadre fireblade
0-1 CS BG squad


1-3 Elites
=9 crisis
=1 riptide

1-3 FA
=2 piranha

1-3 HS
=2 hammerheads - interchangeable weapons 1 with long strike
=1 broadside

3-6 Troops
=30 fire warriors
=22 breachers

Auxillary Formations -
Optimized Stealth Cadre =
1 ghostkeel
6 stealth suits

OR

Heavy Retribution Cadre
1 ghostkeel
2 stormsurge

Retribution Cadre =
Commander
Riptide
3 CS Teams
1 Broadside Team

Command Formation
1 shadowsun
2 ethereal on disk
1 coldstar battle suit
=3 crisis body guards

These 2 units have no place in the Hunter Contingent. You may want to look at running a CAD as well to get these two units in or if you don't want to run the command formation in the hunter contingent.
1 darkstrider
1 skyray gunship

You also have a number of drones - you could run a separate formation detachment and run the drone net.

the plan is to be able to run multiple formations depending upon who i fight. I like to vary my tactics on with them as well so I keep them off balance, we typically play 4 games on weekends and i switch it up my scheme. 1 fight i was thinking of hunter contingent other would be different. the more variety the better, any other units you think i need aside from the ones i planned to buy?

Another Ghostkeel or 2 and 2 more riptides

This way you can run OSC and the Heavy Retribution Cadre or with 3 ghostkeels you can run the ghostkeel wing.
with 3 riptides you have the option to run the riptide wing.

I've had success with multiple sniper teams taking advantage of ethereals powers.

Kroot are good despite what people think. Good for outflanking into enemy back field.