i've just finished a feature doc on machines using mavericks and Yosemite.
most annoying thing for me is that various save and search windows don't remember their last used window size.
they alway open in the default size, which is too small for me.
i believe this issue is not FCP7 specific.
a more troublesome fault is that the simpler eye-droppper doesn't work,
you have to open the colour picker window and use it's eye dropper.
ok for some things, but NOT ok for matching colours in the Colour Correction filters.
so a couple of rather small issues, nothing major for me so far.
all our systems had X and 7
i know when we upgraded X we hid 7 and it's corresponding apps in a separate folder within the applications folder.
i'd do the same thing with X if it is giving you trouble.