I think there is a lot of confusion here.
For example you say non-hierarchical but then later you say
their overall society is non-hierarchical and consists of small egalitarian clans that are bound together for mutual interest and mutual protection from outsiders
This is actually 100% a hierarchy.
A hierarchy in it's most basic from is simply saying A is above B or it's better to do A than B.
Merely placing an action or idea above another.
And any form of society or even intelligent existence will have a hierarchy or sorts.
A predator is pushed by his nature to hunt when it needs to feed. So the hierarchy here is continuing to exist is above expanding effort.
But it also makes tactical decisions all the time.
For example when hungry but confronted with a dangerous group of animals that can cause it harm it might back off because here getting the prey might mean dying.
So I simply don't understand how can you even have an smart creature without that.
Because ask a person in that "low" level society why they are together he then will answer because it's better than being alone.
This is hierarchy.
Now hierarchy can also be based on: strength, wisdom, or speed or whatever.
It's not a fixed thing.
So what is going on there?
You also mentioned
can imperialism manifest from a sense of racial superiority alone
Which seems like an odd point. Imperialism is about power and influence.
They can be built on anything from being power hungry to plain old madness.
But the thing is a society is a complex thing and you simply can't say that every single officer in the British army believed the same exact thing, nor that every single citizen of the Alexandrian "empire" did the same.
So you have a ruling class and then social norms and so on.
But imperialism is about actual power and that is different.
So I guess I'm saying I think we need to know more.
Because like it is said here: you can't have a powerful society with a lot of
Now I think house Telvanni of The Elder Scrolls universe might be something that helps?
And I think what you mean is do they themselves need to work together in order to gain power?
Well. You can build this society as something like 1 family of race A holding power and dominance over races and people.
So you can incorporate extreme aversion to working together.
For grimdark points you can have the leading male, or you can have it females whatever, murder any males offspring to continuing dominating the family.
So a single family with a head will rule lands and hold power over a bunch of people.
How much can that be pushed?
Well. Not as much as if you put 3 families with their combined power together.
They will create a mega family and congrats. You just rediscovered civilization.
However humans are very malleable and once we figured out it is better to cook food than eat it raw we all got in on the action.
Same with erecting huts together and dealing with our smelly neighbors but with the trade off of being protected from the elements and dangerous animals.
But if we assume your race is unchanging in that regard then you can get away with something like I describe.
But intelligent life would, likely, form a loose confederation of families because here is what is going on.
Lets assume your race is very powerful. At a certain point there would be no more land to conquer and no more resources to fight over.
The power of each family would reach it's absolute maximum expansion and by that point the smaller families are done.
Now either risk fighting a war with another major family over a high risk high reward idea such as controlling their land, but also risk another family gobbling you both when you are weak.
Or form a sort of non aggression pact with them and hold you influence in a state of continued vigilance lest a nearby family tries to conquer you.
But before that if a new threat emerged then you will just have to also handle that.
Assume family B have a lot of good farm lands. A human upstart is rebelling.
What to do?
You can ignore it. But it won't be long until that humans controls their lands and comes for you.
You can risk trying to get in on the action. This could pay off and it could actually backfire.
Or you can lend them some help to squash the rebellion and insure that humans serfs know their place.
This is the best I can do.
They don't work together and 1 leading figure holds much power.
But they are not stupid.
However this is a form of monarchy but with less of their own race.
There can be a lot to be said about this but assuming that it even relevant.
Anyway Obviously the world of politics is complex and decisions need to be made.
Cooperation is essential for any empire.
Now magic can actually change that. But it's not mentioned.
And like I said I think we need to know more.
Especially defining what you mean by those terms.