I'm working on a project now with shots in mixed resolutions. Most is shot in 5K, others (slow motions) in 1080p. End resolution has to be Full HD 1080p.
To get the end resolution for output, I am now adding keyframes on a Pan&Scan track for EACH cut to a different resolution-shot, and changing the Scale parameter. This is a lot of work! Is there no such thing as 'downscale everything to 1920x1080'?