First of all, what are the specs of the other SD footage that you need to intercut with your downconverted HD footage? What will be the ratio of native SD footage to downconverted footage in the final product?
If the majority of footage in the project is 29.97 interlaced SD, then I’d suggest making your sequence setting 29.97 interlaced ProRes 422 (not HQ). If the SD footage is NTSC-DV, make your frame size 720×480, lower field first. Then use Compressor to downconvert the 1080p24 XDCAM EX footage to ProRes 422 720×480 29.97 interlaced (lower field first) with 3:2 pulldown added. You’ll need to change the frame rate to 29.97 in the video compression settings dialog and make sure that the frame controls tab is turned on. Choose lower field first in the field order settings, and choose best for the resize filter setting. Make sure to change the resolution to 720×480 with a pixel aspect ratio of 0.9 in the output dimensions section.
If the majority of the project will be the downconverted footage, you may instead want to use Compressor to convert the native 29.97 interlaced SD footage into 24p SD and downconvert the 1080p24 footage to 24p SD, then edit everything in a native 24p (23.98) SD ProRes sequence.
Which route you take mainly depends on whether you want the 29.97 interlaced footage to retain the interlaced look, or if you want it to match the 24p look. In documentaries, it’s not uncommon to see archived 29.97 interlaced footage intercut with recently shot 24p footage (with 3:2 pulldown added). In a dramatic feature, it’s more common to make everything 24p for a consistent look. What’s the nature of your project?
If you haven’t used Compressor before for this kind of advanced format conversion, you really should crack open the manual and read up on what all of the various settings do. It’s by properly configuring the frame controls tab settings that you can really unlock Compressor’s potential as a format converter.
What will be the primary delivery format for this project (DVD, broadcast, etc.)?