how much over 100 should the staffpercentage be?
Is staff automatically fired if a plane is taken out and sold og leased stopped?
See manual for information about Auto-staffing (https://www.airwaysim.com/game/Manual/General/Startup1/#office)
As for your first question: Auto-staffing sets it to 101%, I'm doing fine with it, some players keep it at 100%, some go to over 105%. 101% seems decent to me, which is why I constantly use it.
Question two: staff never is fired automatically. You have to decide if you want to expand in the future and keep it for a while or reduce it and take a good hit on your CI.