Sorry for my English :P

Tamed. Savage diplomats

Evolution of Tamed was accelerated by unknown race by adding innate desire of peaceful behavior. But whoever done it, he done it wrong. Huge, hot tempered creatures still can’t ignore their true nature; they try to follow imposed lifestyle with clear understanding of its unnatural forced origin – as if it’s body illness or addiction – and so with strong difficulty. Impressive success of their foreign policy is based only on efforts of special caste of Voices: to become one of them, person has to underwent an extremely painful neuro-procedure of ‘taming’ all its emotions and reflexes to receive full control over its thoughts, behavior and even body language. It allows to effectively react on negotiations progress and manipulate interlocutor... but since Voices aren’t rulers, ways to resolve domestic issues is complete opposite to foreign policy, and Voices have to use all their skills both on interstellar delegates and own leaders (who are rude and quick for judgment as all other ‘untamed’ citizens).

Faction affinity:

Wolf in sheep's clothing: While Tamed are great in manipulating visiting diplomats, it’s hard to become a real part of Tamed society.

Can’t assimilate Minor Factions. Receives % of allies’ assimilation bonuses.

Faction traits:

Manipulators: Tamed diplomats are gods of imposing an opinion.
Diplomatic agreements and Demands are less expensive / more effective. Additional income from Trade routes.

Restless: Untamed citizens are prone to external influence without clear understanding of consequences.
Increased influence on planet / system ideologies from random events, neighbors etc.

Ethical debates: Despite everything, each citizen is literally forced to worry about ethical side of any issue. 

War Exhaustion is increased.

Let off steam: It seems that protecting your own worlds is the only way to vent an aggression without morality doubts.

Increased attack and defense while fighting in owned (for X+ turns) systems.

I like this concept, the whole unnatural-diplomat thing is intriguing and invites further lore and gameplay styles. Cool stuff. My proposal also relies on minor factions but almost in the opposite way (they welcome them instead of rejecting them). So I particularly like your concept in how different it is from mine and nearly all of the other proposed factions.