Tau can do well in KT, but the big splashy toys are all gone, so you're gonna need to put the riptides away and dust off some of those less used models. Markerlights are all but useless, with them only benefiting one model at a time, you'll need to -really- want that one gun to land to make any kind of ML support worth it, and even then it's probably not.
You can take a basic team of 6 Firewarriors, 3 Stealth Suits, and 4 Pathfinders, with 12 points left, or take a Team of 6 Firewarriors, 4 Pathfinders, and 2 Piranha's with 22 points left for upgrades, and seeing as even the Tau's most basic troop weapon has a 1 in 3 chance of scoring a glancing hit against AV 10 and a 1 in 6 against AV 11. Your greatest enemy will be your generally small squad size, although Overwatch may be fun, especially with "Supporting Fire".
Units by role
XV-8 Crisis Suits
Why do you use Crisis Suits? You use them as cheap, customisable elites, that's why.
With the Basic Crisis Suit Model costing 22 Points, it is more than possible to take a squad of 3 for 66 points, and then stick double Flamers on each for an extra 30 points (96); while this may be almost half of your total points, you've now got 6 Flamer Templates on highly mobile and tough (for kill team) models, essentially creating a literal "Field of Flames"
Or you could take a Single Suit (22), make him a Crisis Shas'vre (32 Points) and give him 2 Burst Cannons (52 Points) for the ultimate storm of infantry killing Dakka. Add a Counterfire Defense System if you're afraid of assault and want to see your enemies cry.
However due to the lack of Squad based rules, avoid using the "Command and Control Node", the "Multi-Spectrum Sensor Suite" or target locks as these will just be deadweights, despite how useful they could be in this situation.
With the use of Combat Specialists, these become downright dirty. 42pts nabs you a Crisis Suit with a Twin Linked Plasma Rifle. Now grab the Weapon Specialist that gives you (basically) Ignores Cover. Pick a model within 24 inches. That model is now removed from play.
Alternatively grab a Twin-Linked Burstcannon with Haywire to mulch vehicles, or grab two Burstcannons and the specialisation that lets you split your shots in order to gun down hordes with enough dakka to make even an Ork blush.
XV-15/25 Stealth Suits
Flat out, these are probably most survivable choice in the Official Tau Kill-Team, with a 3+ armour Save and a 4+ cover save. Stick them in any kind of cover and they'll get a 2+, giving them a better save than almost other Kill-Team-legal choice.
Equipped with a Burst Cannon, and a Stealth Generator, the basic squad comes in a set of 3, for a cost of 90 points, and can take any choice of Support system from the list. It is entirely possible to equip all your stealth suits with Stimulant Injectors, creating a fast moving, High Flying, incredibly tough unit that should be able to handle almost any immediate threat, however you're sinking a lot of points into this, so your Kill-Team will be miniscule as a result. If someone is fielding Guardsmen, JSJing, killing-on-twos, 4-shotting stealths are the stuff of literal nightmares, just don't expect the hit rolls to always go your way.
Run a bigger team of them (5 or 6), with a fusion blaster or two, and they'll have a good chance of whittling down the enemies team to naught without getting themselves shot. Just keep an eye on what you use the rest of the points for, you need to be a little wary of the break tests if you lose too many less-survivable models.
The best reason to run firewarriors is the gun, on a 4x4 table, 30in gets you a really long way, and at S5 it'll hurt most things, including vehicles if you bring enough of it. They're still not gonna be particularly mobile, as the devilfish is probably more points than you wanna be splurging if you've only got 200 to play with, but they're cheap enough and 4+ is good enough to avoid marines gunning them down automatically. Supporting fire still works, so think about how you're spreading your models out on the table, and if you think you're about to get charged try and get a few firewarriors within 6in of each other.
For 60 points, you get 10 infiltrating bodies for board control, all the more helpful if the terrain has forests for you to hide in. Of course, you're going to spend the extra 10 points for sniper rifles, because it's predictable wounding regardless of what your opponent brings. And while you wouldn't want to assault with them, they're at least no slouches at close combat. Pairs well with the stealth team above for a fully infiltrating kill team, and you can use the kroot to bait a charge that a burst cannon and counterfire equipped stealth suit gets to cut down. Kroot hounds won't do much for you here, but Krootox give you a multi-wound model with a board-dominating gun that still infiltrates.
Markerlights find much more limited use in KT due to squads being split into individuals. Pathfinders are still worth considering though for their access to rail rifles and ion rifles, both of which are capable of killing most infantry in KT, and providing some anti-light-vehicle support as well. EMP grenades are also an option here if someone's bringing a hard-to-deal-with vehicle like a walker. (Then again, you should really be thinking about ways to shoot the damn thing instead).
Piranhas are great in kill team, where the bigger vehicles don't make an appearance, and really killy anti-tank stuff is less common as well. AV11 gives you decent resilience to troop weapons if you watch your facings, and a good turn of speed means you can scare the inevitable enemy light vehicles with sneaky side/rear facing shots. Combine this with the free gun drones, which can either give you a decent amount of extra firepower, or instead be dropped to provide more JSJ-hijinks or objective grabbing and the piranha becomes pretty fantastic value.
Gun drones are the only thing really worth talking about here, marker drones suffer along with pathfinders with markerlights being way less useful, and shield drones not shielding anything other than themselves. If you're up against anything with a 5+ armour save, and have even a little bit of LoS-blocking terrain on your board, JSJing pulse carbines will annoy the hell out of your opponent. Just be aware they can die relatively easy if you don't get them hidden, and that might hurt you when break tests start happening.