Alas, no Miguel, the thumbnails are done for with the QT7, since it is the deprecated movie player. The only saving benefit of using it is the non-conversion of videos.
QuickTime X is now nested into the OS and the entire framework for video is a different set of instructions. In effect, QuickTime 7 is treated the same by the OS as VLC is - as a third-party aplication - so it's not got the integration with the Finder.
As to the Avid codecs, the issue *may* lie with the installer, it did a double-start on me which is a sign that it's a 32-bit program.
I got an install to take with Pacifist - http://www.charlessoft.com - and it loaded them into the Library/QuickTime folder, though that is not where the Perian codecs are, so that looks like it is the default for the QTX player.
Since I do not have any Avid hardware of files, I cannot try, but PTO214, could you give it a shot and see what comes of it?
If you have the codecs installed and they don't work with QTX, you may want to try moving them to the same QuickTime components folder that the Perian codecs went into, which would be the System/Library/QuickTime folder. Here's a pic of both so you can see..
http://home.comcast.net/~foodandart/MacForums/QuickTimeLibrariesLocations.jpg
I think at that point, you'll be using them with QT7.
As far as QuickTimeX and the whole thumbnails thing, I have a sinking feeling that Apple is closing off a lot of the OS to make it more iOS-like, so a lot of the nifty functions people use and expect will only work with the 'official' Apple-sanctioned applications/filetypes. 😟 Booo!
Good luck and let us know what you find!
Regards,
Deb.