You could do that, a mix of things might be interesting.
If you want to show the might and ruthlessness of the Reticulans during the height of their empire, perhaps one of the races once rose in a revolt. After the revolt was crushed, the Reticulans claimed their homeworld, gave other choice worlds to those who had been loyal. They forced the disapora of the natives to other, unsettled worlds that were deliberately marginal (but not impossible) for the survival needs of the race in question and with practically no colonization support to make them unable to be a threat to the Reticulans for a long, long time.
Some minor race might actually be quite prosperous and powerful, like they've totally "integrated" at least on the surface, and are a quiet "model minority" ... who own a lot of businesses, make a lot of money, and have a lot of political power because of their wealth, but their race primarily exists in enclaves, they hold few worlds of their own, and where they do make settlements, perhaps they're famous for renting airless moons or orbits to set up habitats or domes and are famous for taking some airless rockball of a moon and turning it into a prosperous tax-paying community of miners, merchants, and manufacturers. Unlike many "prosperous minorities" that are looked upon with suspicion, they're quite open and inclusive - they freely make business arrangements with others, allow others to settle in their enclaves, and so on. Of course, the core of their civilization might not be so open, but they've decided that not making waves is the best way to prosper and survive.
Some could be mysterious. "On the far border Cicek marches, it is known that there are the Ssellassians. Surprisingly little is known of them, save that they've had some conflicts with the Cicek, who have an indifferent opinion of their fighting prowess."