The idea of learning technology from captured settlements got me thinking. When you capture a Maskling settlement, shouldn't it stay as a Maskling settlement? Just because you own the place doesn't mean that the Masklings forget how to train rat riders.
So idea: settlements are tied to race. The majority population of a settlement determines its race, which determines what structures can be built in it. So when you capture a Maskling camp, it stays a Maskling camp and you can build Maskling structures in it. Over time the population will get used to you ruling them, and will loose their slave status, then you can create settlers to found new Maskling camps.
This would be similar to the way the ancient Persian empire functioned. Conquering people, and just replacing the ruler, leaving other aspects of the society intact.
Or, if you're so inclined, you could migrate humans into the newly conqured camp, until the population flipped the race of the settlement. That'd be a more Roman approach.