Yosemite is version 10.10.1, the first bug fix was a month after release. 10.10.2 beta was released to developers, today.
Reverting to Mavericks is a non trivial project, unless you create a bootable backup with SuperDuper or Carbon Copy Cloner.
Creating duplicate of your Mavericks install on a partition or hard drive and upgrading that backup would be safest.
The new (today?) updates to FMP 13 are partially for Yosemite compatibility.
Macintouch has reader reports on FMP versions and Yosemite compatibility.