It's ok to rotate the cartridge. Rotating the compressor housing is the same, you just loosen the bolts a bit so the compressor housing spins freely on the cartridge. Considering I've had it totally out of my car, I know the deal.
The turbo is three distinct parts, the turbine/compressor housing, and the cartridge. The housings are held onto the cartridge by "clamps." The bolts in the housings tighten down on the clamps, which hold the housings tight. The housings are sealed to the cartridge with thin little gaskets. Even with the cartridge completely seperated from the housings, it shouldn't leak at all, unless one of the seals is shot.