The only races that come to mind are the Orks, Reek and the Arachen which we know that both of those pose a lethal threat in the extreme for all other life in the Galaxy.
The Tau were designed as good guys, the one guy in a Cthulu horror story that doesn't know he's in one.
Yes they manipulate societies but nothing I would ever infer as 40k level 'evil'. Yes Aun'va got... Hitlerish... But his appearance was only after a loud minority kept declaring the Tau were too goody two shoes. That doesn't disregard it- but I think you'll find a lot of members on this site agree that those eras weren't entirely thought out. Personally, I'd say the Tau are morally somewhere between 'hitler' and what the founding fathers of america thought about nation building.
Now the topic was the enclaves; I know that they are now able to use Kroot and vespid, and they have a human world, but I can find nothing on their stance on integration of humans. I have hope of good lore on this somewhere because Farsight is merit-based and as a result non-fire caste can use battle suits. So if some human showed the discipline and skill could they be integrated into the fire caste or would the humans forever be forced to use the human auxiliary rules found in IA:3? I.e. Lasguns instead of pulse rifles?
And you won't. There frankly is none in regards to integration. The entirety of Tau culture is merit based. Farsight is unique in that the Fire Caste is entirely in charge. Usually, a 'Chia'gor' council is held to determine which Caste should lead in an endeavor- as Kakapo mentioned from the first codex, the real 'fear' of the Enclaves is a Tau society on a constant war-footing.
Humans have been integrated into Tau military to a point of relative seamlessness since the 3rd Sphere under Shadowsun. They are armed with carapace armor, communications suits and pulse weapons all in line with the Fire Caste. There are examples of human heroes, human units committing to one another with the Ta'lissera, in short, they are more like Tau than they are human in many ways.
We also have Gue'vesa'O Va'deem, a former inquisitor left behind during the Damocles Gulf Crusade who is still alive leading a counter-Imperial intelligence initiative that pilots a customized battlesuit. So that's canonical.
I'd say if you wanted to model some figures- you're totally within fluff norms... as weird as that is to write, lol.
I think it's ultimately an aesthetic. If you want to model humans using and piloting Tau gear- it works. If you want to do IG allies, it also makes sense, especially since Tau manufacture human weapons for auxileries like in the Taros campaign.
Game wise alone- you're stuck with the Taros version of Gue'vesa.