Should I create a Fusion Drive before or after downgrade (from Yosemite to Mavericks)?
Your first idea is the only way that it can be done.


Boot into recovery HD and create a Fusion Drive (erasing all data)
Install Mavericks on SSD part of Fusion Drive


A Fusion drive is for all intents and purposes a single drive, the OS sees only one Volume. The promise of a Fusion drive is that it will optimize the contents in such a way that you will receive the highest performance possible while having a large volume to store your data in.