Hi Lachlann,
As a former owner of a 8" newt + EQ5, you will definitely need two counterweights to get a good balance. An extra 5kg will definitely be more than enough to balance your system
Even if you added cameras, finderscope, upgraded focuser, additional wiring, guidescope etc it would still be fine.
Yes if you have two counter weights, then they both would sit further up the shaft (if you double the number of weights, then you would need to move them up the shaft up by half). If you can wait then the weights can be had cheaply second hand here
good luck!