It isn't just balancing issues I'm wondering about. What about size?
Now, I love Star Wars, but I'm not IN love with Star Wars, if you catch my drift, so I only know what the movies, games, and occassional Wookiepedia search have told me. So I'm not entirely sure as to the size ratio between an ISS and a planet, an ISS and a SSD, and an SSD to a planet.
However, I did just think of one other thing:
The Death Star.
We could put it in the Cruiser's section and make it rediculously expensive, or make it stationary and have it be like the Novelaith (sp?) cannon for the TEC, or something like that.
Hell, why not do that with an SSD? If we could solve both balancing and scaling issues, why not put it in Cruisers rather than Capitol ships and make it amazingly expensive?