I am working on a project with multiple video clips from youtube and they are all HD in 1280x720 or 1920x1080 mp4, when I look at them, they are in different frame rate and FCP starts to crash when i import multiple clips with different frame rates, then i was using compressor to compress them into ProRes422 (HQ) but the frame rates remain unchanged and FCP still crashes.
You could try going into compressor and create a setting where it changes the clips frame rates to a specific frame rate instead of a setting that keeps the same frame rate... Still seems like an add glitch.
I've been using MPEG Streamclip for this kind of thing - do "Open URL..." (copy/paste from the Youtube page), then "Export to Quicktime", putting the required frame rate in as well as choosing Uncompressed 48k for the audio, size and codec, etc. That way what I get on my computer is clips that drop straight into FCP with no issues.
[Andrew Rendell]" I've been using MPEG Streamclip for this kind of thing"
I experimented with MPEGStreamclip and JES deinterlacer to do a frame rate conversion. Both were quick and looked similar with JES being slightly better. Compressor, using frame controls did a slow but better job. If you do the frame rate conversion at the same time as the transcode to ProRes422, you should get the best result. HQ is overkill for H264 (mp4) source material.