mix left shifter and font derailleur of different speed
It will work OK while you should not mix 2 &amp; 3 speed (MTB and road) front gears. 8 speed chain is a little wider than 9 speed and the FD will be a little wider but it's not much wider and will move your chain pretty nice. It's not affecting the shifter cable pull and FD move.
The problem in mixing 8 &amp; 9 gears is in rear shifter.