You should not have to worry about breaking FCPX on Yosemite; the more serious question will be how badly it breaks the older FCP7 an FCS3 apps and quicktime pro. SOme pathfinder users have reported early success on that but I will remain cautious, heck, I didn't bite on Mavericks yet, myself. If the system is stable and reliable as-is, it takes a lot to get me to upgrade. I don't do it just for the sake of being up to date. There has to be a compelling reason, and some minor GUI tweak isn't it.
I'm using the final Yosemite public beta and I haven't had any problems with FCPX, though I've only done basic work in it. I had been keeping an eye on compatibility reports and ran a test instance before I went whole hog. I also saw on twitter this morning that Alex Gollner (Alex4D) had FCP 7.0.3 running on Yosemite.
I think that "some minor GUI tweak" is selling it (or any OS update) short. The compelling reasons to updating to Mavericks and now to Yosemite have outweighed the downsides. Currently on Yosemite I'm finding continuity very useful and I appreciate the broader access to 3rd party extensions in sharing and such. It's also been very speedy.
If an upgrade breaks needed functionality, then it isn't much of an upgrade. But if that's not happening, I look first to the benefits that it offers and am willing to give it a go (test instances and backups are good!). And functionality is different than "this is how I've done things", it's about end results. There are times to not mess with workflow, but the unexamined workflow is kind if like the unexamined life (a trite cliche?). Where can I improve by seeing things differently? [insert FCPX comparison here]
[nick ring]"Alex Gollner (Alex4D) had FCP 7.0.3 running on Yosemite"
I understand 7.0.3 has been running in 10.10 since very early betas. My guess is it will continue to do so until QTKit (currently deprecated) is finally disabled. At that point FCS, QT7, and anything else depending on that framework will cease to function. Anyone who uses these apps should either freeze a system now, or find alternatives. :-)
In searching this question re FCP7 (yeah, I know), I did see a post noting that several people had problems with FCPX. The solution seemed to be to uninstall the app, delete the preferences and cache from the library, and then reinstall the app.