Copy and Paste 720p onto 1080i timeline

I'm compiling two seasons of shorter bits into a longer series for cable. Season one was shot and captured at DVCPRO 720 and 2 was captured at 1080 ProRes 422. They are multi-video layer timelines and I'm needing to mix and match bit of different format onto the same timeline. When I do so, some of the material is off sized and some is ok... it's sort of a mixed barrel, even after rendering. Is there some trick to combining different formats on the same timeline?

Yes: don't. Batch-process everything to your timeline format before assembling it. Final Cut handles the occasional off-format shot just fine, but trying to mix lots of off-format material is a nightmare.