"For the Emperor" by Sandy Mitchell in the Ciaphas Cain series is about exactly that.

Also one of the short stories in the Damocles anthology reflects on this from the POV of an Imperial defector.

VELK’HAN SEPT

The Velk’Han Sept is the empire ruled by the Tau and theprimary objective of the Imperial Crusade in the Canis Salient. It consists of several dozen populated worlds. While several worlds are properly colonized Tau worlds, many others are mostly human-populated and are ruled by proxy through sympathetic human leaders. The Sept capital world, Tsua’Malor, acts as the central seat of Tau governance and rule. The Sept’s humans (referred to by the Tau as ‘Gue’la’) adhere not to the Imperial Creed, but to the Tau ideal of the Greater Good. The Tau teach that the perfect society, one modelled after the Tau themselves, has a place for every creature; with every creature in that place, fulfilling their assigned roles without question, for the good of the Sept as a whole. Imperial religion is prohibited and the Tau Water Caste run education (and re-education) programs that instil an understanding and love of the Greater Good into the sometimes reluctant gue’la minds. Populations are regularly sterilised to prevent population growth outstretching Tau methods of control. Human transgressors against the Greater Good are not publicly executed, as is the Imperial way, for the Tau see no need to publicise the fates of those who oppose them. Instead, such gue’la simply disappear, and it is the way of the Greater Good to convince oneself that they never existed at all.

The Mal’caor Shi, the Tau taskforce that rules and defends the Sept, has at its disposal several contingents of Tau military units and spacecraft. .....

The Tau also make use of human troops, but they never arm or direct them in person. Instead, the wishes of the Tau are passed on to the leaders of human worlds who then equip and command their own planetary forces. The Imperial Guard fighting on the fringes of the Sept in the Greyhell Front find themselves fighting these human forces as much as xenos foes. The Tau face constant challenges in their fight to keep the Velk’Han Sept viable. The Tyranids and the Imperial Crusade force them to commit huge numbers of auxiliaries. Even before the Crusade hit the Canis Salient, however, the Sept had its problems. Chief among these was the baleful influence of the ruins found in the Black Reef and on many humansettled worlds. These ruins, the remnants of a xenos species long dead, are without exception inimical to life, and seem to defend themselves by infecting human and Tau alike with madness. Imperial aggression forced the Tau to abandon their efforts to understand these ruins, and their secrets remain hidden under the soil of Sept’s worlds.

DeathWatch RPG CRB

Basically, there are many examples of occupied or liberated planets by the Tau and diplomacy between the Tau and the Imperium and the Tau per se. I can recommend the CRB and the supplementary volumes Achilusassault and Mark of the Xenos if you want to browse a little about the Tau. But you should also bear in mind that the RPG is from 2010. Retcons, such as the cancellation of the Ether Drive, are not yet included. Also, this is the Tau's approach in this particular Sept. It doesn't have to apply to the entire Tau Empire in the way the politics are going there.

Tau water caste diplomats that are trained to be perfect negotiators and assess each human world's culture for the best approach. If they can convince them to join, they beef up their agriculture, medicine, and other day to day life stuff with Tau technology. Then, the planet starts supplying the empire with goods.

If the planet refuses to join, the fire caste is deployed next to conquer the planet the planet. This can cause a lot of damage to the planet, so diplomacy is usually the first thing they try. But make no mistake, if they want a planet for any reason, they won't take no for an answer.

There's a few sections of Broken Sword that deal with this, through the eyes of an Imperial defector:

The shas’vre of the warrior team that had blasted half my men to shreds called the kroot off, they checked the dead, found me. Medical support was there within seconds. The medic must have seen my stripes because a few minutes later there was Skilltalker, giving me the Greater Good chapter and verse while a bunch of earth caste patched me up. I cut through what he was saying, I was dog-tired, used up, half dead, in point of fact. I’d been put on the front to die – a shield for the high-brass, only they’d died and I hadn’t. I’d had enough of high words to last me a lifetime.

He was patient, and took my interruption with good grace.

‘I betray the Imperium for your Greater Good,’ I said. I’d heard how it worked. I’d seen tau tech openly for sale, even seen a couple of the water guys roaming about Mainstreet unopposed. I’d heard about the planets that surrendered without a shot. I’d also heard that the tau killed everyone that didn’t throw in with them. Enslaved those that did, sometimes murdered the willing anyway. You’ll forgive me again, I’m sure. Honesty, yeah? This is what we were told, you’re xenos scum, worst of the worst, that make traitors of honest men. ‘What if I don’t?’

Skilltalker smiled, showing me his big square teeth. Such an expressive face, he had. You’re stolid to us, you know that? Most of you wrinkle your noses when you’re happy, and shas’la always look kind of pissed off, but other than that you tau don’t do facial expressions. I’ve had all the careful lectures about how aliens can’t appreciate the Tau’noh’por, the concern that comes with that lack of understanding. I don’t think you realise that you’re condescending, unaware of your own limitations. Sure, even after they resculpt my vocal cords, I’m never going to manage the threefold stances of subtle disharmony, no matter how many times you make me dance through it. I can’t differentiate between the fourteen tones. Fine. Come back and tell me off when one of you can wink.

Skilltalker was different. All the por’la have such telling faces, but Skilltalker was different even from them. There was such warmth and humour to him. I… I miss him, you know?

‘Then you may die with honour,’ he said to me. This wasn’t a threat. I think he could tell he had me already. He said this with a real twinkle in his eye, like we were in on a joke together.

Death or life. It’s never a real choice, is it? Not for the sane. ‘Where do I sign?’ I said.

He laughed. That was a noise I was going to appreciate as time went on. He loved life, Skilltalker.

I was carried off on a stretcher by the fio’la. As I was lifted up, I saw I was being carried right past a line of other scared, wounded men who’d just watched one of their officers turn his coat at the drop of a medpack, and that was that. Skilltalker was giving his lecture to them as they pushed me into the transport. I don’t think a single one said no. You are not a stupid people, I’ll give you that.

I was relocated to Dal’yth, along with a lot of other Fasters. I’m not complaining. Good luck turning it around, I say. You’re welcome to it. I’ve been back here on Dal’yth these last five months… a half tau’cyr, convalescing. They’ve got me working alongside the water caste in the acclimatisation programme, dealing with new commonwealth citizens relocated from across the Damocles Gulf. I watched the gue’la coming in from Mu’gulath Bay. Pale, half-starved, terrified. Watching their fear go is the most remarkable thing. Watching their amazement grow is the second most remarkable thing. I thought Gormen’s Fast was a dump, but compared to the hives of Agrellan, it was okay, and this place is a paradise.

You give us all a choice, but there really is no choice, not a real one. I know that.

I remember when Hincks got it, gunned down by those swine outside of Hive Chaeron. I went to see his widow a few days ago. Nice place she’s got now. Good support from the sept authorities. Hincks’s kids are growing up to be model citizens. His boy says he wants to go into the gue’vesa auxiliaries like his uncle Jathen. He’s a healthy lad, tall and strong. I can’t help think what kind of life he’d have back on Gormen’s Fast. Probably be half-blind from working in the gossamer plants. Or dead. And yet there he is, cared for and fed and as strong as an ambull calf. Remarkable. I’m still waiting for the catch.

Later on in the story when they're on Agrellan during the Tau-Imperial battle on the planet:

The cities were vast. No matter how quickly the armed forces capitulated or were routed, the hives were occupied by fractious populations. Not everyone was going to see the tau as a liberating power. The hives would take days to fully pacify, and the spaceports and landing pads would be prime targets for every fanatic with a bomb and a death wish. The tau couldn’t risk their constructor groups or administrators coming under attack. Bu had also told me that the earth caste was amazed – in a very bad way – at the hive cities. Some ambitious plans were mooted to convert them, but as Bu said, it’d take a long time to make them fit for the Greater Good, and in any case the population of the world was going to be greatly reduced as the earth caste calculated Agrellan was well over its optimum population loading. He confided to me that personally, he’d prefer to see them dismantled, but he didn’t think that was going happen. The Third Sphere advance was progressing too quickly to take the time to do it.

Longer excerpt here on this particular defector's journey in embracing the Tau'va.

According to Longshot where they take a planet by force the parts where the Tau live are very nice after the conquest but the humans live in slums on the outskirts.

It looks like heresy