Electronic components rated at 5V probably would only start
complaining at 2x the rated voltage, so you've usually got some
fat to play with @ 8V (fully charged pack?).
However, we are also talking about BRUSHLESS motors - their
speed isn't directly proportionate to the voltage fed to it.
Double the voltage and the RPM don't double... (but torque does).
(I run 12V 4" brushless fans at both 12V & 7V... the speed isn't
very different either way. But the other day my finger got stuck
into one when it was running off 12V - oww that hurt. Sliced a
chunk out of my fingernail!!
At 7V it don't even hurt.
Dare say your ESC should be feeding it the right voltage eg 5V.
Can't help you on the rules, though... how anal is your techhies?
What if you buy a servolead Y-adaptor, you can stick that on any
of the servo or ESC wires and run the fan off that??
You need it anyway if you need to keep your BATT plug free to
run a AMB PT or Orion lapcounter at the same time.