I should point out that these new city types are unlocked at the same points as Cities, which for Carbon based life is the Planetology tech, and Silicon life gets them from the beginning.
They are in addition to the standard City. I didn't want to have to update the whole ImprovementDefs.xml file every time a new patch came out, so I couldn't change the City. I wish there was a way to replace an individual improvement, but I don't think there's a way to do that.
treborblue is asking for what kind of modding tools you want him to make.