You can have your various types of asteroids that do different damage in the stellar abilities file if you want. But apparently to get it to work right you have to disaccociate the stellar ability type from the size of the asteroid. Doesn't really do what you wnat, I know, btu that is all we can come up with right now.
We had a similer problem adding new types of planet sizes. Not exactly what you are doing here, but a similer symptom. We were able to work around that eventually. Something might come up for this too after we think about it a while.
Geoschmo