I've been playing as a custom civ (synthetic) and can confirm that the pop is growing on some planets. It must be a pretty low value though, as planets with a population of 1 or 2 don't grow.

they were all ones i colonized.  I first noticed it with a planet i had settled with 2 pop.  When i went to shuttle over another 2 (several turns later) it had a pop of 5.  Out of 12 planets 3 were growing their own population at a higher than normal rate (don't recall exactly what it was but it was ~1.0 every 4 or 5 turns)

There's a difference between synthetic and silicon based.  Silicon (Slyne, Onyx) grow at the same rate as carbon based, but require durantium instead of food to build cities.  Synthetics like the Yor aren't supposed to grow by themselves, there are colony projects you use to increase the population (i.e. your robots spend time building more robots).  Synthetics don't have population caps, so they don't need cities.

Never played Robotic civ....so got that wrong.