[apache]

I really don't find it unrealistic that races can colonize any planet type if they research it. If you want to base it on something like a race is born on a solid, rock planet, they should easily be able to adapt to an ice environment, since it is essentially a different type of rock.
Gas colonization obviously brings up quarrels with gravity since gas giants are more massive, and hence have a much larger gravitational force. But then again, we have already abandoned mass effects with the propulsion systems, so why not allow a similar line of reasoning with gas giant colonies? My thoughts are that this type of problem is exactly what would be researched anyways.