Seems a bit extreme. You should have some long-term consequences, yes, but permanent 0% happiness implies that nobody will ever assimilate. In truth, I'd expect most purifier pops to mellow out eventually once they're no longer exposed to the government's propaganda and can see for themselves that the xeno menace is not actually out to eat their children.
That's absurd. I'm a human myself, and I don't desire the complete extinction of all spiders. I don't want to touch them, but I don't kill any I find. I know that some people even keep tarantualas as pets. And that's with completely non-sapient spiders; throw in renowed holovid personality Spider Joe and his beloved comedy sketches, and you can get even further.
You know what else humans have a visceral reaction to? Physical deformities. It's an evolutionary instinct that keeps us safe from infections. When I was growing up, there was a kid in my class with a cleft palate. To us, he was just a funny-looking kid. I might have thought he looked creepy at first--I was young enough that I don't remember when I met him--but the entire class was able to love the guy regardless of any ingrained instincts.
Besides, it's absurdly impractical to go through every species portrait and rank it based on how repulsive it should be to each other portrait.
Traits are probably the closest you can get to things that should cause actual friction, and even then you're oversimplifying things. Decadent pops would certainly prefer a xenophobic authoritarian government if they could be in charge, yes. In an empire where they aren't primary, on the other hand, decadent pops would prefer xenophilia, as that at least gives them a chance to become elites.
And you're leaving out the traits that should naturally incline pops towards "nicer" governments, as well. Nomadic pops, for instance, would likely prefer egalitarianism, as that gives them the best guarantee of freedom of movement.
Of course there should be nicer traits of naturally nice species, the conformist, docile, etc...
But you'd be dead wrong about the bugs, you might be a very crazy exception to the rule but the overwhelming majority could not suffer the thought of even looking at a giant bug like that, let alone the thought of touching it, they'd rather die, hell, it's very common for women to actually freak out and throw whatever they have in hand at cockroaches the instant they see them, including other people, some species have absolutely no chance of ever, ever being compatible, even if a tiny minority can dehumanize themselves and pretend to be cool around them while holding back either terrifying fear or absolute disgust.
I can easily see humans being compeltely incompatible with bugs, possibly most crustaceans and fungoids, and I'd imagine lithoids would feel better around plantoids and fungoids as both plants and fungus probably grow on their bodies naturally, but everything else would probably start looking too alien.
And, as I said, there is absolutely no chance a psionic fanatic spiritualist pop from an ascended empire that understands souls could ever tolerate the existence of machine intelligence, specially living among them, they'd be considered a mockery of everything they see as sacred, something worse than Jim Crow to americans.
Also, there is no such thing as "mellowing" a genocidal pop, they come from an empire that would make the real life nazis look like the nicest people in the galaxy, they do not even view aliens as people, they are filth to be cleansed and nothing else, imagine human terrorists that kill themselves for their beliefs just to take out as many of the "other" as possible, now realize those are 2 human groups which are very close to each other, and now imagine something that isn't even an animal, isn't even from this world, and has no hope of ever seeing anything other than filth in front of him, be it becuase of his biology, or society. They should pose a real internal threat for xenophiles as the moment they enter your empire you'd be forced to make hard choices, or live with the permanent consequences, they do not engage in diplomacy, they do not make deals, and they do not integrate, at least, they shouldn't.
To think that "we all" want the same things, have the same goals, and have different ways of reaching it is a very human perspective, aliens, by definition, do not share this, they do not even want the same things you do, they do not see what you think is moral as moral, they do not want to ever agree with what you want.
Here's another example: An insectoid collective that doesn't look too repulsive somehow integrates in human society, except they are fundamentally different from human psychology, they do not even understand the concept of indivuduality and live for their collective, so they often see a human behaving in a way that doesn't help society, like littering, and they just go and kill that human, then you judge them, send them to jail, none of their species care, because individuals do not matter to them, they see another human littering and kill them again, and again, and again, what do you do? They can't even understand why you'd be angry about this, and they do not care about human forms of punishment, until you punish the collective, their collective do not care about your justice system.
Even worse, the moment they decide they had enough and that humanity, as a collective, is harming their society and what they view as moral they start attacking all humans as they see no difference between one criminal or an entire group of people, how do you react to this? How do you integrate a species with such alien ideals? They don't even have to be hiveminds per se, but they'd surely feel much more confortable living with pops from one.
Currently, what we have in Stellaris are humans using different alien skinsuits.
Edit: How do you think a species that abhors the idea of killing any other kind of life and lives off sunlight, radiation, filtering or whatever else, would feel if they got to meet humans and their meat/vegetable consumption? Do you think they'd just shrug as they walk into their new super markets filled with corpses of animals & plants as people naturally chat about what they want to eat today? Do you think that'd be just a "cultural misunderstanding" they could clear up after a few thousands of years?
What about that parasitic funguns species growing out of the teddy bear? What if you heard them chatting about how they enjoy the fuzzy feeling they get when they start consuming children, particularly, as their cognitive capability is lower and therefore they feel much more fear and pain in the process, a process that's required to their survival, how many years do you think it would take to assimilate that species so we could send some of our young so they could "assimilate" with them to survive? Or would you rather give them another specie's babies instead? Which species would a xenophile egalitarian utopia chose as sacrifices to allow these aliens to live?
Or you walk into a planet and find out it's the fungus vs teddybears, they live there, they've been fighting for survival against each other, one needs the other to survive, and the other is trying to defend itself, they can't survive without a parasitic bond with a sentient being, which is extremely painful and condemns the person to be a permanent slave within their body, while alive, they are both "people", which side would the xenophile egalitarian utopia help? Or would they just turn around and pretend they didn't see that planet?
What about Necroids/Necrophage?
