Hi there,
I just did a clean install about a month ago for all my editing apps having moved to mountain lion,
For FCP7 You must do a clean install (with Apple Discs) or it will not work. I tried DMGs wont work…
FCP7 seems to be working fine it does have a few issues but that would due to 3rd party plugins as far as crashes are concerned. I even was surprised to see a pro app update from 2010 appear in my app store!
I would recommend trashing thumbnails every other day or if you get a crash.