Never tried but my for the federation integrating klingons, romulans, cardassians and so is:
- first liberating them with my ethics
- vassalizing the new empire
- intergrate them

Heres how I get them to be willing participants

1. leave them as vassals until you have the following technologies, dont be in a rush, they wont rebel against you, they rarely occupy more than their home world and a couple of outposts, so focus growing south and south west, same thing goes for the Flaxion.

2. Techs you need, pleasure done, and holo combat areana, mind control laser

3. look at their happiness bar and see which policies you can edit to appease them

4. early on once you get trait points modify your race to get rid of your slavery disadvantage

5. end slavery, build all of the above items on their planets, within 50 or so years they will be loyal green bar subjects with the same ethos as Romulans.

@ themanwiththehat - when you liberate a planet the government switches ethics but the pops do not. They may eventually switch (but probably only on the vassal capital planet) but it would take centuries as the ethics divergence is not very strong, and the AI doesn't use edicts AFAIK.

@mwhite2300 - good ideas. I still try to roleplay with the romulans, and since they had reman slaves I'm assuming they were not averse to having other slaves. I'd love to play them not as federation 'light' but it is difficult.

I've had a mindcontrol laser + re education camps + information quarantine going for at least a century and am making minimal progress.

@syntax - You said "I'd love to play them not as federation 'light' but it is difficult." When a totalitarian regime controls your state through domination that is not exactly "Federation light", there are sever consequences if they choose not to follow orders, like having your planet bombed out of existence.

"I've had a mindcontrol laser + re education camps + information quarantine going for at least a century and am making minimal progress." What you are missing here is that happiness is the key to ethics change. Ethics % is partially a formula of how happy the pops are, so if you want them to change the biggest factot is happiness. Make them happy by changing policy(which you can later change back), having a governor (5* gets you 10%, a leader wwith happiness bonus 10%, pleasure dome 10%, and holo combat arean 5%, by ending slavery you may net around 16% depending on the pop, economics from planned to either free market or plutocracy can net 5-7%, stopping resettlement can add up to 15%, so on and so forth, just look at the policies you have it tells you the % change)

"and since they had reman slaves I'm assuming they were not averse to having other slaves", I have big problem with this, notice the Remen are not enslaved when the game starts, thats because even in the Star Trek movie that features them, they are not enslaved, though it is assumed they are not free to leave either, the Remen in Star Trek lore are a semi autonomous offshoot of the Romulan race that evolved differently on Remus due to the harsh conditions of said world, they are not slaves. Even the mine masters that order them around are also Remen.

Also no where else in Romulan stories or shows, have the Romulan ever enslaved anyone, obliterated whole planets? Yes. The klingons on the other hand very clearly enslave other races, just not Klingons. And Romulans have never enslaved their own by any evidence.

The Romulans may in fact be in favor of some degree of racial genocide, they have a superiority complex for sure and dont play well with others, but some how they do well enough to trade...ala Romulan ale.

I personally think the Romulan ethos needs a rework, eliminate the slave trait, change materialism to the lighter version of the Borg genocide trait.

Thanks for your reply.

Perhaps a missing ingredient is happiness..? I've never played vanilla stellaris, but before posting here I'd read that low happiness actually speeds the process. I observed that high happiness does indeed counter high ethics divergence. The posts I read suggested that it was actually better to keep this pops unhappy whilst exposed to a high negative ethics divergence.

I agree with you RE the Romulan ethos. Decadent is a weird trait, and I presumed it was there because I was 'supposed' to play the Romulans as slavers. Further their ethos does not support advancement down the Military Republic government chain.

I agree with your assertion, I also dont understand that, especially because in previous versions of this same mod you could, this is a recent development, Devs, is this unintentional perhaps?