Firstly, 9 speed is not just 8 speed plus another cog - the cogs are closer together so you cannot just use the 9 speed shifter with 8 speed cogs.
Pre-2001 9 speed shifters will be compatible with the rear mech, after 2001 you need a later type rear mech. Post-2001 shifters have the number of gears on the front.
Campag further complicated things by changing the hub splines for the cogs - 9 speed will not fit on 8 speed splines (which are shallower) and you have to swap out the freehub body. 9 speed saw a switch to aluminium bodies and needed deeper splines to stop the cogs chewing them up.
The shifters are rebuildable, so you could fit 8 speed shifter innards into your 9 speed shifter - Mercians hold most of the spares. Lots of 8 speed shifters vanished as they were upgraded to 9, hence the relative rarity.
In short, it's simpler to stick with 8 speed, otherwise you will have at least to change the freehub and possibly also the rear mech.