officially supported FSSO subsystem version for all components (agents) is stated in FortiOS Release notes and usually include just latest FSSO released alongside with FortiOS.
However, FSSO protocol is stable and so evolution is mainly about new features while old ones are kept backward compatible.
So regardless it's not official, almost (as there were surely less perfect versions) any 5.x FSSO will most probably flawlessly work with any 5.x or 6.x FortiOS.
Upgrade of FortiGate do not necessarily mean to upgrade whole FSSO (in a hurry).
If your FSSO was working OK, it will almost certainly do still work after upgrade of FortiGate.
However I'd also suggest to schedule maintenance and sometimes upgrade FSSO as well. Do not run to situation where you'll leave FSSO as is for years, just upgrading FortiOS.
Tom xSilver, planet Earth, over and out!