I think you desperately need to include some measure of planet size - currently, a tiny Chunk planet has exactly the same population characteristics as the largest terrestrial (gravity being the only exception to that). Since small, cold, low-G planets are the most common, and there is no population penalty for planetary smallness, races that live on such planets can easily outnumber all others. Seems like a pretty serious exploit.
Not really, as a lot of people could live on a very small planet. Consider the population density in major cities on Earth, or the population of India or China when there is still a lot of room left in those countries. You also still have a finite limit on population due to growth rates so even if there were masses of habitable planets, you wouldn't have enough people to colonize them all. Besides, this is only an issue if you set your gravity tolerance really low. Finally, who is to say what level of population density is acceptable to a particular species, or how large the individuals of an alien species are? Those would have to be considered too. If someone wants to work out the numbers regarding a realistic population limit on a small world, using humans as a basis, then I might look at it but I suspect those numbers would be a lot higher than you think.
Steve