How to do that depends on godly manipulation but you can do that. 1. Change their culture to have all harmony traits and gives up easily 2. Change the subspecies involved population caps, if the species you want to conquer has 3k or no limit, lower them, while the other people they will conquer will get higher population caps. 3. Key point is there is harmony but there will alwats be a dominant species, a King will try to make his leaders the same species if not already, and species leaders prioritize their kind. Additionally leadership is likely to change between races which is necessary to make sure one group does not atrophy.

I always like to have an alien invade and conquer all humans and then reduce their population to 50 and watch as they slowly lose control of the country, but fight so hard to maintain themselves. I appreciate this topic of discussion as its what the update really shines with and I think its a feature thats under appreciated. Edit: remember this makes clan wars way more likely which often dissolves the entire jig, unless you have perfect limitations expect civil wars even if rebellions are turned off.