I have been thinking up new empires for my next playthrough when Utopia comes out, I'm not sure how you could roleplay fanatic authoritarian/xenophiles... You LOVE making aliens slaves?
Okay, I'm going to take another pass at this. The respective ethos' receive as bonuses..
Fanatic Authoritarian:
Allows caste system species policy.
Connot use democratic government forms.
Can displace aliens.
Empire Modifier:
Slave Unrest : - 20%
Resettlement costs: -50%
Xenophile:
Can always give aliens full citizenship.
Increased opinion to other species.
Cannot use full native interference policy.
Cannot use the no refugees species policy.
Empire Modifier:
Trust Growth: +25%
Diplomatic Influence cost: -25%
I'm still seeing this as a dynasty focused feudal monarchy, like the Hapsburgs. Your government exists in the form of competing noble dynasties that are more than willing to marry into the households of aliens in order to forge alliances, business partnerships, and land rights. Their willingness to do this and the additional opportunities it presents accounts for the increase in trust growth and the reduction of diplomatic influence cost. The non-ruling upper class tend to be valued retainers of the nobility, picked for their skill and abilities, the ones hired by the royal family would be your leaders, your governors would be the heads of great family's with enough influence to sway and control the actions of the world or sector they govern. There is very limited social mobility, owing to the authoritarian structure of your society, but it all still runs well enough. Your lower castes are land-bound serfs, your energy merchants and scientists mostly live in large urban centers where the ruling class are still nobility, but the rules are a bit different owing to a wealthier and better educated underclass.
To take on a vassal is to grant a minor title to the leader of some foreign land in exchange for his oath of fealty. In most cases, it's a handpicked leader of some nation you conquered that didn't used to even have a monarchy or had foolishly done away with theirs some time ago. You incorporate him when your society has put in enough influence and effort to make it inconceivable for the leadership of your vassal to maintain control of their society without being the sworn subject of your great kingdom.
Playing like this, only directly conquer or vassalize other authoritarians. For others, use the liberate planet option to create vassals that share your ethos, and if you can't do it all in one chunk, add the rest of that nation to your vassal in subsequent wars. Only incorporate your vassals once their populations have settled down enough that taking them over won't give you a huge supply of spiritualists, or militarists, or some other group of undesirables that will destabilize the crown.