First of all, always cool to see detachment rule proposals, so thanks for sharing your work! Second, I haven't actually picked up the tau codex, so take what I say with a grain of salt.
1. Recon Cadre:
Detachment Rule Co-Ordinated Ambush: Units with the Recon Keyword (Darkstrider, Firesight Marksmen, Pathfinders, Sealthsuits, Ghostkeels, Piranhas) are all selected as being a guided unit to any observer unit that also has the Recon Keyword. In addition, they do not receive -1 Ballistic Skill when shooting a non-spotted unit.
Pathfinders gain the Battleline Keyword.
I'm not sure what the part about guided/observer units means. Do you mean that you can select multiple units to be guided by a single observer? Do all units with the right keyword get guided automatically when a single unit with the right keyword observes? And if so, isn't that basically the same as giving all recon units +1
BS with no drawbacks?
I'm usually iffy on handing out Battleline as a mechanic because it tends to either not actually do much or to functionally do to much, but I think it might actually work pretty well with pathfinders here.
Generally, I like the concept of a sneaky, mobile detachment. I'm just not sure what Co-ordinated Ambush is meant to do.
2. Unity Strike Force:
Detachment Rule Strength of the Greater Good: Kroot units gain the following ability: While this unit is within 6" of a non-Kroot unit, that unit gains the Stealth Keyword. Non-Kroot units gain the following ability: While this unit is within 6" of a Kroot unit, that unit gains the For The Greater Good Faction Ability.
Stratagems and Enhancements: These would focus on using an auxiliary and Tau proper unit in tandem to greater effect. Bonus to Kroot melee if shot by a Tau unit, or Kroot gaining Fight First if they Heroically intervene to protect a Tau unit. Stuff like that.
Again, I like the concept. Love me some alien auxiliaries buddying up with their tau overlords. I really like the concept of strats that focus on actively helping each other out as well.
A couple criticisms:
* I'd really like for vespid to get some love here. Ideally I'd like more alien datasheets in the tau codex in general, but that's outside of the scope of this project, so in the meantime, rules for vespid would be nice even if it's only a strat or two.
* On that note, and this might be too complicated, it might be nice to let this detachment ally in some imperial guard so the gue'la can join in on the fun?
* Something feels off about the detachment rule. A 6" aura on a unit of kroot is a massive area of effect, and kroot generally want to be moving forward to chew on stuff rather than sitting back and shooting. So I feel like this detachment rule sort of encourages you to gunline up and use your kroot blobs as cheap Observer units/Stealth generators for your assorted suits and tanks rather than encouraging the kroot to actually act like kroot.
With that in mind, maybe do something like:
Enemy units within 6" of a kroot unit suffer -1 to-hit on ranged attacks made against non-kroot units.
^ So instead of awkwardly encouraging your fire warriors to get near the front lines or encouraging your kroot to sit back and pretend they're stealth drones, this encourages you to send your kroot forward and do what they're good at, thus keeping your tau in the backfield safe. I'm not sure if you need a rule going the opposite direction (tau helping kroot) as general fire support and the fire warrior rule to debuff enemy shooting already kind of complements a kroot advance.
3. (Couldn't think of a good name for this one):
Detachment Rule Advanced Targeting Systems: Once per Phase for each Tau unit, you may re-roll one hit, wound, or damage roll made by a model in that unit.
Stratagems and Enhancements: These would focus on the non-suit Tau vehicles, like the Hammerhead, Skyray, Devilfish, and aircraft.
I'm afraid I don't really like this one. It basically just rewards you for running vehicle skew by giving you a flat kill-more-better rule. For the same reason I don't love a similar rule on my eldar, I don't love this rule here.
If you want to make a vehicle detachment, I'd encourage you to think about what it is about that army concept you find interesting and what about that army concept isn't well represented with the current rules. Where possible, write detachment rules to make an army feel *different* instead of *stronger*. Maybe this could be the armored cavalary detachment? Provide rules that focus on infantry utilizing transports rather than on just hitting harder? Maybe this detachment grants sticky objective rules to certain units so that you can lean into the mobility of your skimmers instead of standing on no-man's-land objectives all game? Just spitballing.
I'm sort of blanking on other detachments I'd like to see. Once upon a time, I'd have liked a drone detachment that let you toss out protocols (doctrines for drones), but most (all?) of the drone units are legends now. Looking at the detachment rules for Kauyon and Montka on Battlescribe, I feel like those might be kind of underwhelming. Maybe the strats and enhancements save them, but the detachment rules themselves make it seem like they just give you kill-more-better rules in either the first or second half of the game. Instead, I'd rather have Montka handing out Precision and giving Oath of Moment style bonuses to certain targets (preferably with some sort of drawback) while Kauyon grants bonuses against enemies you fell back from earlier in the turn. Actually change unit behaviors rather than just giving them Lethal and Sustained hits.