1. I like playing a subterranean race because of the bonus to population. When building androids on a planet with a high maximum population, I'll often receive a message about how there's "no room" to build more, even though the planet still has room for six or seven million more people. Is this a bug, or something related to how androids work? (I can provide a save that illustrates this, if needed, but it's fairly easy to duplicate.)
Androids naturally have the tolerant special, but not things like subterranean or aquatic. Population of different "races" (which for our purposes androids are, here) can only occupy up to their own limit - if you stick a race with no specials on an ocean planet with aquatics, for example, the population of that first race would not be able to grow past the normal ocean limit, while the aquatic colonists they share the planet with would be able to increase to gaia levels.
Because of this, androids can usually be added to a planet when the natural population has hit cap, because they fill slots their own "tolerant" special provides. But if you have the right combination of specials and world, your native population may have a higher cap than just tolerant would allow, preventing androids from being added there.