Best way to edit mashup video of downloaded movies? So I wanted to do a mashup video for personal usage, meaning scenes cut from downloaded commercial movies or Youtube. This is typically relatively highly compressed h264-material.

The workflow that I attempted, but feel is not optimal, is to:


Remux losslessly to .mov containers and stereo-only audio where applicable
On 2x speed, watch through in QuickTime Player X. Whenever I want to use something, I use the Trim function, Duplicate the trimmed movie, save the duplicate, and then undo the changes in the original and continue.
In FCPX, I import the clips and put them in a project.


The problems in my workflow is, that it would be great with a tool that is more optimal to scan large amount of video (e.g. a full length movie) and mark sections to become clips. Secondly, FCPX frequently crashes when I try to use the clips natively. I have converted some of them to Optimized Media (Prores 422).

Is there a better tool, or a workflow combining tools, that makes it particularly easy to do these types of video mashups?
Not on FCPX, but on Avid.  However, this is what I do and it may help:


Download at the maximum possible resolution you can download at.
Use AVIDemux as my rough-cut tool to mark 'A' and 'B' and save as clips with meaningful names.  Leave a few seconds on either side. 
Import clips into Avid


Sometimes, I have to convert the container to 'MP4' or resample frame rates and AVIDemux handles these well.