I will have to check the resolution of the planet textures. I thought that may cause it as well. I am convinced however that the DX3 or DX5 DDS format is behind the error. I hope that is the case, and not a resolution problem, as I think I used the same resolution as vanilla Sins planets use.
Also, with regards to meshes, I have been successful in making different sized planets and stars, so I may be able to help you there. A lot has to do with the glossiness of the model. Try to keep it the same number as vanilla meshes. Any variation will cause weird effects. Also, if you make the planet bigger, then you usually need to make the txture bigger as well.
Also, you must include both the light side and dark side textures as well, or again things will look weird. If you want to work together on this, I'd be happy to help. Send me a PM and I'll give you my email. Then you can send me what you are working on. I know you use XSI for modeling, and I use 3D MAX, so there might be an issue there as well. I think together we can find a solution. I leave it up to you.
Again, thanks for posting. It means a lot to have your support.