I've tried it on 3 different computers running Yosemite and I get the same error every time. I've even tried 3 different variations of FCP. The official physical copy with the disks, it just crashes on me, and 2 other non physical copies.
After doing some more research I think it may be because i already have FCP10 installed the installer thinks I already FCP. Going to try to delete FCP10 then install 7