Kroot as an officially recognized army don't really exist, outside of using the few Kroot units in the T'au Index to make your armies (and even then, I think you're missing out on Kroot
HQ).
Kroot, however, are an excellent base to make just about ANY ARMY CURRENTLY PLAYABLE (with perhaps the sole exceptions being Necrons and maaaaybe Admech) with a lot of converting (or if you can rely on large numbers of lightly-armored infantry). They are also fairly adaptable as representing multiple different types of armies with the same models: An Orkroot's toughness, and Aeldakroot's speed/dexterity, and a Tyranid's ferocity can all be represented by the extremely adaptable kroot bodyform, and the Kroot have had alternative ammunitions in the past. Combine that with kitbashing and reasonable inferences representing "krootified" weapon loadouts, and just about anything can be represented (though some, like ultra-high tech Admech and Necron tech, might seem inappropriate).
Tau-like Kroot ("Kroot'au") just need kroot to eat enough T'au corpses (for the greater good) that they get comfortable wearing/using T'au tech. They're otherwise fairly similar, baseline stat-wise. Toss in the occasional Kroothereal, and you've got an awesome theme.
Ork-like Kroot ("Or'kroot") aren't just a speculation, they've existed in fluff for quite a while now. Given different Kroot clans focusing on different prey, some of the Ork-like specializations could easily be represented.
Eldar-like Kroot ("Kroot'ari") have been also been encountered in the fluff, including their natural psyker capabilities. They're also already somewhat fitting for the Kroot as they exist - fairly fast, lightly-armored, best in skirmishes, and somewhat naturally frail. Each subtype of Eldar could be reasonably represented, with different subfocuses (Drukari = riptear focus, Craftworld = speed focus, Ynnari = carnivore-focus).
Astartes-like Kroot ("Kroot'artes") could be a combination of both bloodlines, or independent of either. If you go with the former, you can represent more melee-focused chapters (Space Wolves, Blood Angels, etc.) as heavier on the "Ork" side, more disciplined/balanced chapters as more even mixes, and the fastest/most-psychic chapters as heavier on the Eldar side. This easily expands into the flavors of Chaos as well.
Tyranid-like Kroot ("Kroot'yranids") also have some basis in the fluff, generally with a Shaper laying down the law and declaring them inedible (the horror!) after the inevitable results set in. A sufficiently dark take on Ethereal's control over information and personnel gives all the wiggle-room needed to get around some pesky Shaper's declaration, and Kroot already infected might escape to breed more. They'd make excellent Genesteealer Cult representatives for substantially similar reasons.
IG-like Kroot ("Astra Krootarium") make less sense,
IMO, but could make a fairly excellent baselike Kroot Mercenary (lots of Imperial gear looted, as that's incredibly prevalent), perhaps with some Orkroot Mekrootic support providing the armored support. There are already hints that Kroot tech comes from earlier incorporation of Ork DNA (oldish fluff, though - Xenology is fun but oddball), so passing that on (and converting other aspects into Knarlocs and Krootox and Kroothounds to represent sufficiently large/armored bodies) seems reasonable.
Seriously, Kroot have some of the most flexibility in what they can represent, both on biological and technological levels.
Because if you're going to make a Kroot army like the Kroot Merc army lists of old, you'll want/need to do a metric shitton of converting anyway. Why wait for an official list when you can borrow the mechanics of other armies in the meantime AND have it make reasonable thematic sense?
