[Marty Ward]

I think that you could move those 4m around indefinately.
Once you get a majority of the correct breathers on a planet you can queue the maximum facilities and replace them with any other type of pop. I don't think you lose the buildings.
It works this way if you capture a palnet from a race that has advanced storage, once the facilities are built they stay unless you try to scrap and replace them, then your current limits come into effect.