Just finished a project with pretty much the same situation…
I’d personally just edit in a PAL SD timeline and let FCPX work it’s magic. The 1080i will look fine. The NTSC will look a fraction soft but from my experience it’s not really noticeable – even projected.
If the NTSC video is noticeably poor quality then you might run it through compressor before you start editing and use the frame controls to enhance the conversion – but you’ll be adding a lot of time, maybe hours, to the conversion process.
It’s amazing how often just letting Compressor do its thing yields great results 🙂
Jeff Kirkland | Southern Creative Media
video * audio * post * production
Melbourne, Australia