You could convert all the NTSC movies first, or convert them on final output to MPG2. I'd limit the number of passes you make doing any sort of compression/re-sizing to the least number possible.
I'm not sure compressor is going to do the best job at resizing, but others on the list might be better able to answer that.
Also...depending on your source material, converting from NTSC to PAL could introduce some motion artifacting since you're resizing and changing the frame-rate. If the source is really high-quality and doesn't have a lot of camera motion, it might look very good. If it's marginal quality and there is a lot of camera movement or even movement within the frame, it could end up looking kinda iffy.
We have had good luck using Nattress Standards Converter right in FCP. It is a unique workflow so follow the steps closely but you will have good results. Export the PAL clips and encode in compressor...compressor will see that theya re PAL and encode accordingly. AC3 encoding does not change.