usually rogue planets do not orbit stars but travel around until they are caught by another star and then orbit that one (for a while possibly, until they are "swinging out" (which takes millions of years to happen possibly) of orbit again travelling further to the next star, whenever they reach one). Maybe I should add that to the description.
well, there's a reason why on class N planets usually there aren't more than 1.5 bn people on there. They are living in special habitats, that's also why terraforming class N planets takes 3-5 times longer than terraforming class H or P planets. You have to see that surface components and matter on those planets are just not class M type and a terraforming ship cannot convert all molecules on the planet's surface like billions of years in planetary evolution does (microorganisms that produce bio-molecules etc.pp.). If there's only sand on a planet, then there is just sand, no matter if the temperature cools down to 20° Celsius and also if water is being pumped up to the surface. There is still no good soil for microorganisms to turn the desert into an earth-type planet in let's say a 100 years time. They don't work that fast and because we don't want to switch classes in-game by terraforming we don't let any fancy future tech that pimps up microorganisms to be fast enough occur
. Also, since nearly all races are humanoid and like it earth-type, there's no need to allow turning class M to L or M to P and vice versa.
People colonizing for example a class H or a class P planet need to use fancy tech to get enough water and food supply, mainly by using ground water. That's why these planets have much less growth rate than real class M planets because it takes longer to implement a sustainable water and food infrastructure on those dry planest. In case of really dry deserts with no ground water at all, we are in the case of class C planets actually and those dead planets have less than 1 bn people on it anyway. In the end, a class H or P planet isn't that far away from class M in terms of ground water but it is far away in terms of chemical elements on their surfaces, especially the biochemical ones which determine class and look of a planet.
The info-text on planet classes need to be revised I guess.
A galaxy creation editor just like good old botf had is planned. In bote.ini you can already set star density and minor race density percentage-wise. Star density means stars per sector and minor race density minors per star system (values are percentages). Good idea to make planet type density choosable too. Will have that recommended.