It's bothered me that a few species appear so often in the galaxy while others don't. I was just poking around and i realized it is more than the luck of the draw.
Based on about a dozen test galaxies: r 4730
1) A species with "CanProduceShips & CanColonize" but without "playable" will never appear in the galaxy.
2) A species with "CanColonize" listed before "CanProduceShips" breaks that and all following species.