How do you populate a colony? Moving billions is impracticable; giving birth to billions takes hundreds of generations, or then some very severe birth control policies (say, 30 children per mother).
Also, millions of Earth-like planets? Trek has plenty of those, granted, but supposedly only because our heroes choose to visit those rather than non-Earth-like ones. In "Metamorphosis", Kirk exclaims that "we" are on a thousand planets and expanding. Whether that's us the humans or us the Feds, it's orders of magnitude below your estimate. And sort of realistic.
Assuming Romulans started with 10,000 colonists, what would their population be in 2000 years given modest 2% growth rate?