Turning this feature 'off' does this also effect other aircraft?
Yes, of course, since it's a general GSX setting, and it's not per-aircraft. It simply mans GSX will ask for confirmation of the number of passengers it has estimated, so you get a chance to correct it.
It's a feature that has been asked by many users, since the GSX estimate might be wrong with airplane in a Cargo-Pax Combi configuration (747s, MD11, etc.) but, since it's not possible to know the "type" of a payload station being either passenger luggage or extra cargo, it's not really possible to do a correct estimate of the passenger number automatically, in a Combi configuration.
This could be solved, if airplane developers would use the GSX custom variables documented in the GSX manual, like the FS Labs A320 does. But, since we cannot expect all developers would do this, we added this feature, so you can direct tell GSX how many passengers the airplane is supposed to load.
And, because some users might be annoyed by the extra step, or don't used airplanes with a Cargo+Pas Combi configuration (so they never noticed the passenger number estimate to be wrong), we haven't set this option as being enabled by default.