First... a tip for group battles, either have your opponants use a single force organization chart between them, or take two yourself. I find sometimes in team battles, sheer number of enemy heavy support choices can be too great (but then, most of my team games were vs. IG and Eldar... your mileage may vary).

HQ - Winged Hive Tyrant - 2x Scy Tals, Wings, ADGLWS, tox sacs and warp field - 182
Infantry crusher, will be hugging terrain for much of the game.

A good Tyrant and a good plan. Eat some troops every turn and he'll be good.

Elites - 1 Lictor - 80pts
Will be used for ambushing necron squads, may drop him as may not be of much use.

Drop him behind the Destroyers or near the Stealth Team and tear them apart. Lictors are great against heavy weapons / low model count units.

Elites - 6 Warriors - ADGL WS+I, Tox Sacs, 6x Scy Tals and 6x RCs, leaping, extended carapace - 258pts - Meant as my slow-moving synapse, will be thrown at the necrons due to the rending claws.

Nice brood that also works well against the Necrons, good for dodging that rapid fire. Big enough to wipe out squads, while small enough that you probably won't destroy them on your turn. Beware that Necron Lord.

Troops - 11 Genestealors - ext carapace - 220
Troops - 10 Genestealors - ext carapace - 200
These will probably be thrown at the necron side of the army, to deny the WBB rolls.

Good plan, good Genestealers... and they get saves against the Gauss Flayer. Thumbs up.

Troops - 15 Spinegaunts - 75pts
Troops - 15 Spinegaunts - 75pts
Meatshield. Purely and simply.

A meatshield with better initiative than fire warriors. Stick them in CC with the T'au, or gun them down and watch the fun. Beware O'Shava.

Troops - 14 Hormagaunts - 140pts
Troops - 14 Hormagaunts - 140pts
Meant to engage the enemy Tau lines as quickly as possible, will still do a fair bit of damage.

Hormagaunts are almost overkill, but the extra charge range is nice. Excelent anti-Tau broods.

Fast Attack - 6 Winged Warriors - 6x Scy Tals, 6xRCs, 6 x Wings - 228
My fast Synapse, back-up for my gaunts.

The 'Gaunts are going to be primarily used against the T'au... consider 4 warriors with 2x scything talons, and 2 with the rc/st combo. You won't need too much rending on the T'au side of the board, and those extra attacks will definitely help.

Fast Attack - 3 Raveners - 3x ScyTals, 3x RCs - 108pts
Will be along with my stealors in going after the necrons

I cringe... not because I dislike the Ravener, but because Necron Gauss weapons deny them their saves. Consider using them against the Tau commander instead, or go hunting for their destroyers / lord... but try to stay away from the normal troops, if you can.

Heavy Support - Shooty'fex - 1x BS, 1x VC and enhanced senses - 148pts
Heavy Support - Shooty'fex - 1x BS, 1x VC and enhanced senses - 148pts
My anti-tank/firewarriors. I didn't add ext cara due to the amount of AP2 weaponry likely to be thrown at them.

Nice 'Fexes, cheap effective bombardment. Use them on the destroyers first, then concentrate on other threats (like the Piranha). Expect him to deep strike his gun drones and assault your Carnifexes with them (to keep them from shooting for a turn or two). Spread them out so he can't completely negate your AV firepower this way.

I'd almost suggest dropping the Raveners for hive tyrant with Warp Blast for anti-Necron work, but right now your list looks solid, without being so devastating that they'll never want to play against you again. Have fun (and teach the T'au player that "uber" isn't everything... I've got a friend just like him, except he played Eldar (until we convinced him to try Daemonhunters, an army that's powerful but much harder to min/max)).

 

Thanks for the comments.

but right now your list looks solid,

I was slightly worried I might be slipping, having recently embraced Slaanesh (almost completely different tactics and playing style).

either have your opponants use a single force organization chart between them, or take two yourself.

We tend to use one between us, in a sort of informal way (as in most of our army builds don't seem to exceed more than half of a FOC anyway, especially when they only have 1000pts each to work with). This was also sort of to discourage my Tau opponent from fielding 3 HQs (he normally plays Blood Angels and would quite happily field 3-4 HQs in a team-game army, and the HQs he builds take a significant portion of your army to take down).

The 'Gaunts are going to be primarily used against the T'au... consider 4 warriors with 2x scything talons, and 2 with the rc/st combo. You won't need too much rending on the T'au side of the board, and those extra attacks will definitely help.

Yes, that makes more sense. I'm used to fighting armies in power-armour, hence the sheer amount of rending in my army, I'm not used to fighting 'normal' troops.

but because Necron Gauss weapons deny them their saves.

Ah, the beauty of Ravenors, quick enough to make it to where I need them. Hopefully by the time they hit the enemy lines the necrons will already be engaged in CC (may throw a unit of hormagaunts against em to bypass the rapid fire).

I've basically made the changes suggested to the winged warriors, giving me enough points to add an additional genestealor and 2 spinegaunts.

I'll add a brief overview of how the battle goes.

 

Well, I won. Wasn't convinced I had for the first two turns, but managed it.
My Tau friend fielded the predicted force, but my Necron friend decided to surprise me....with a monolith...

Turn 1 - They managed to grab Turn 1, and blew large chunks out of my gaunt squads. The stealth teams and kroot were infiltrated, and knocked a wound off my carnifex and Tyrant. My spinegaunts broods were decimated by unnaturally lucky shooting.
My carnifexes retaliated by destroying the pirahna and scaring the monolith, a squad of hormagaunts charged the kroot and a squad of spinegaunts (well, the survivors) charged the stealth team, supported by my other Hormagaunt squad. The Kroot and Hormas managed to kill roughly equal numbers (about 6 each), and the other gaunts killed one stealth team member.

Turn 2 - More deaths, the monolith unleashed on my other spinegaunts squad, slaughtering them to one gaunt, and then warriors opened fire on my now suddenly exposed ravenors, killing one. Some maneuvering of my Tau opponent, and several gun-drone shots into my winged warriors, killing 4 and wounding another. The Stelath team-combat continued, losing literally nothing and the kroot-hormi combat resulted in a casualty each side.
My hive tyrant then joined the hormi/kroot combat, my ravenors charged a nearby warrior squad, and my winged warriors joined the stealth suit combat, rending them to death and leaving everything exposed...

Turn 3 - The gundrones and Devilfish opened fire on the hormis/winged warriors, slaughtering all but the warriors, who the gundrones (foolishly) charged. The hive-tyrant/hormaguants slaughtered the remaining Kroot, and consolidated into cover (a large building).
The Lictor bursts out of cover into a firewarrior squad, and slaughters 2 before taking a wound. The carnifex takes a shot at the devilfish, and destroys it. The other carnifex takes a shot at the destroyers, and kills one (before it WBBs). My Hive Tyrant then flies over the building and into combat with the destroyers, slaughtering them all. The Ravenors kill a few more warriors, and the winged warriors kill a few gundrones.

Turn 4 - My Hive Tyrant is meltaed/fusioned/rapid-fired, and reduced to one wound. My Tau opponent, scenting blood, charges into CC. Warriors open fire at Genestealors, and pick off three of them. The monolith picks off one of the walking warriors.
The Ravenors are joined by a squad of genestealors, and annihilate all but one of the squad of warriors, The Tyrant destroys one of the commanders shield drones, and survives the counter-attack.

Turn 5 - The remaining warriors open fire on the genestealors, killing another 4. The remaining warriors in the Ravenor combat are slaughtered, and the ravenors consolidate into the Tyrant combat. The winged warriors kill another gun-drone.
The Carnifexes open fire on the monolith (failing to do any damage), and the stealors charge the necron warriors and lord, killing enough of them for the army to Phase out.
The Tyrant and Ravenors reduce the Tau commander to 1 wound

Turn 6 - Firewarrior shooting knocks another wound off the carnifex, before barbed strangler shots from both fexes cause them to flee to the board edge. THe Tau commander is killed, and my opponent surrenders...

Was a bit hit-and-miss, and my walking warriors were totally useless, but managed to good victory.

 

Lucky that Necron player brought the Monolith... I can't imagine the devastation if he'd fielded a squad of Immortals instead.

A good battle, it sounds like, and a close enough victory to keep them playing. All in all, a good learning experience for two new players.