Does the optical sight which is fitted to the Stalker Pattern Silenced Bolt Pistol (First Founding Wolf Scout) and grants the weapon the Accurate quality count as a sight as mentioned for the Weapon Upgrades (a gun may only have one sigt) or can a Stalker Pattern Bolt Pistol be fitted with another sight from the Weapon Upgrade section?

A sight is a sight is a sight is a sight.

So yes, it counts as a sight, and as a weapon can have no more than a single sight you can not add a second sight to the pistol.

iirc, it seems to work like a telescopic sight, except minus the long/extreme range penalty decrease. Which makes sense for a pistol anyways when you think about it.