Sure - the basic concept of a multi-ethnic social organization presupposes that ethnicity doesn't affect how you react on social conditions. In the 19th century and early 20th century, it became fashionable to say that only nations of one ethnicity could ever function. There are, however, many centuries of empires functioning with a myriad of ethnic and cultural groups living under the same law and administrative structure, and it works fine, because people are basically the same everywhere. If they weren't, it wouldn't.

Again I don't see how that follows and you can state it with such certainty. Have you got evidence for your assertions?

P 1. A policy is a set of procedures a state employs to get a desired reaction from a population.

P 2. An empire is a state that employs some policies across its entire territory.

P 3. Multi-ethnic empires with strong cohesion have existed.

So if you take 1, and 3, you see that for both to be true, people have to have very standardized reactions to policies across ethnic 'lines'. I state it with certainty because it's obvious.

Doesn't follow. Why do they "have to"?

Counter-example: people within a single ethnicity have a huge range of reactions to policies (based on all sorts of things, whether or not genetic).