Another question: What file types does everything have to be in (i.e. model file type, animation file type, etc) and where (exactly or appoximately) are the planet files located?
The planet files are in GameInfo, called Planetxxx.entity. PlanetTerran, PlanetAsteroid, PlanetIce, etc. Basically all the game data is in GameInfo. Abilities, research techs, planet bonuses, buildings, ships, etc.
As for the rest, there's a bunch of different stuff. Models are converted to the Sins .mesh using the ConvertXSI tool in the mod pack.