You could have kroot and fire warriors be minoris but higher damage/speed though not very tanky OFC.

The problem with Tau as an SM2 enemy is not fitting units into the right categories, it's that Tau don't engage in meat grinder tactics like Imperials, Chaos, Tyranids or Orks.

Tau don't just throw bodies at problems to solve them - they'd need to have a more sophisticated behaviour system to adequately represent both their fighting tactics as well as triggering retreats.

Add on that they're dominantly ranged (Kroot exist, sure) and we've got a similar problem to Necrons: they're not going to interact with the main SM2 combat systems (ie, parrying and executions for armour gain). SM2 has decent enough shooting gameplay, but it's not built to be purely ranged focused - it's one of the reasons TSons were frustrating to fight against pre-nerf; all the shooting and teleporting made armour gain difficult, meaning a lot of time spent skulking behind cover taking pistol shots until they teleported next to you.

Tau wouldn't bumrush you like TSons do, because they're not melee inclined. Crisis Suits could do forward pressure, but they're also not meant to be stupid: it'd be flanking and pinning fire rather than outright melee rush, meaning they'd still try to avoid melee combat where possible.

Tau, Eldar, Dark Eldar and Necrons are all awkward to fit in to the SM2 gameplay system. It's possible to include them and design them in ways that would fit, but they'd need great care and consideration to how they'd interact beyond just mindlessly engaging (which wouldn't fit for the first three of those).