Hello. If I shoot a film at 1920x1080 24 on the 7D but then need to make a slow motion sequence, can I shoot it at 1280x720 60 and then be able to edit them seamlessly? What are the steps I need to do to mix these? I've heard 1920 30 is just not as nice as 60, but I worry about the different aspect ratios. What does one do to combine the two and also will there be a quality loss on either one since there's a size difference? Thanks!
Sure, it'll work. Just shooting at 60fps isn't enough to recreate a slow-motion sequence, however; you have to interpret the clip as 24fps, which will give you a playback rate of 40% of the original shooting rate. Select the clip in the bin, and go to Modify > Interpret Footage, and set frame rate to 23.976 (not 24...).
In the sequence, use Scale to Frame Size (right-click on the clip and select this option), or select the clip and go to the ECP and use Motion > Scale to enlarge the clip. Like any enlargement, you'll lose a little quality in the process, but I do this all the time and it looks fine. Sometimes a little bit of the Sharpen filter can bring back some perceived resolution.
As for what to shoot, what is your final deliverable. In other words, is there a client? Is it for web or tv? Me personally, I would shoot and finish at 1920x1080 and deal with a little quality loss on the upscale. But of course that depends of the percentage of footage shot at each frame rate. The final deliverable. etc. etc. So I guess we need more info.