I saw Battle Beyond the Stars, at one point I thought about using SFX from others movies (I think there is also a battleship design real close of the Tantive in BBTS if i remember well. Or was it Star Crash?) but in the end I felt that not many shots from those movies could have been used for what I had in mind (exept maybe some explosions). And I wanted to do a Star Wars edit, not a mash-up of several movies anyway.

Mixing new music with Williams score is tricky, indeed. But replacing the whole music is almost imposible because there is still a bit of Williams score in the center audio chanel (the one with most of the voices and sound effects).
And the audio programes I tried to remove the music also removed some voices frequencies and ended with robotic voices feeling.
I spent quite some hours replacing every laser sounds on the bridge scene and play with audio volume to insert the voices back. Don't ask me to do this on the whole movie or I jump through the window, with a wilhelm scream ! ;)