Anyways, I have seen people try a number of things: Grab some cheese units from other codices, and add them to our army list (Vendettas, Scythes, Paladins etc.), units that do what we do (shoot), heavily armoured assault units (TH/SS terminators, paladins again, Nob Bikers), disposable wounds (orks, IG) and psychic heavy hitters. While they all work in their own way, what we really want is synergy. I have tried allies from a number of different codices, and drawn certain conclusions about them.
Space Marines: Ah, our imperial battle brothers. Popular for several reasons, one being that everyone and their grandma has a SM army, so simply adding it to a tau force is only natural, right? Admittedly, some of their units have great synergy with ours (scouts, sternguard) and others are good for filling niches tau can't (pushing into their deployment zone and keeping them out of ours, ASM and either variety of terminator works). There are really two units I want to give a little bit extra emphasis to however: Librarians and scouts. Librarians are good for all the obvious reasons, yes, but also have some interesting tactics when mixed with our army, particularly broadsides. Three power really stand out here: Force Dome, it isn't a great invul. but it makes the broadsides less dependent on cover and so increases tactical flexibility. Gate of infinity is great for several reasons: Get broadsides out of harm's way, get side/rear armour shots, and last minute objective grabs in Big Guns Never Tire. The other one needs to be rolled for, it is the biomancy power 'Endurance'...FNP, IWND, Relentless broadsides? Yes, please. Scouts are great because they have the range of tau troops and so mesh with our play style, have sniper weapons which is now a big deal, and are a cheap way to fill the troops slot. I usually add a lascannon stormtalon to round off the allied contingent here.
Eldar: Don't mesh with our style of play very well. We are mobile, they are fast. Big difference. Their speed means they will get near the enemy, and we won't be able to keep up. Unsupported eldar=dead eldar. The good things, though, are that they have divination so can boost our shooting, eldar jetbikes so they can have T4 farseers keep up with our crisis suits and JSJ with them, monstrous creatures if that's your thing, and can have jetbike troops. So while you can play around with them to make them mesh, you will really just be shaping them into a tau list with different models, so really what is the point? Farseers are all that comes to mind.
Necrons: Lots of people are trying them thanks to us having access to cheap nightvision, and them having their night-causing rules. Or for their flyers. Both are not the greatest reasons. Most armies have access to nightvision, either in the form of searchlights, blacksun filters or simply being dark eldar, other armies either us short ranged fire or are assault oriented. Nightfighting isn't that big a deal. As for their flyers, a lucky volley of bolters could down them. Pass. I am sure there are other ways to run them with tau, but they are not immediately obvious, or appealing.
Grey Knights: It is tempting to take Draigo and a bunch of paladins. Don't. If you run grey knights alongside tau, I would recommend either a henchmen list with psyfleman dreads, or a PA list, because warp quake really is a great rule for an army that does NOT want anything coming that close to it, especially at full strength, having not crossed the board while getting chewed by pulse and burst cannon fire. Librarian would be pointless for the most part, what with us not being battlebrothers, however it could be useful for psychic defense or buffing other GK.
Chaos: I see potential here. The new chaos codex can really create just about any army type, which is good. It also has access to some ridiculously tough units, and if you run out of slots for broadsides, take obliterators! Being so new, I will need to playtest, I am however thinking of either a daemon prince with some oblits and cult marines, or a Tzeentch chaos lord with the sigil of corruption, 2+/3+ with a terminator unit attached, standard marines as troops and again, oblits. Bonus since people haven't figured out how to counter it yet, due to newness.
BA/SW. These are both gimmicky codices. If you use either as allies, you may as well just take their cheese units. I wouldn't recommend them though unless you REALLY want a runepriest in a drop pod with some GH.
IG: Same as above, don't mesh too well, being simply a "brute force" kind of army. If you take them, may as well load up on vendettas and hydras I suppose.
DE: Wouldn't suggest it. Suffers from the same drawbacks as elder, don't have battlebrother psykers to compensate.
Daemons: Could be fun, can't see it working out. Would be hilarious to have An'ggrath fighting alongside the tau too.
Well that pretty much covers specific armies. Not to sound like a broken record, but I will repeat myself; strive for synergy. A force that works in a way similar to the core force, yet accomplishes things it can't and covering some of it's weaknesses, is what you should be aiming for.
