We can conjecture - and admittedly, that's all it is - that so-called "single race" empires such as those mentioned did, in fact, conquer and subjugate new races as quickly as the UFP made allies a/o members of new races. In fact, there is direct evidence - the Remans we saw in Nemesis, who were explicitly described as a subjugated race/servile caste, surely didn't spring into being fully-formed and enslaved instantly at some point between FC and NEM. The fact that we didn't see those races previously merely points to the fact of their subjugated status, not necessarily of their non-existence.
"We've been over this. We don't shoot first and ask questions later."
"Of course! We never ask questions."