The auto is really a fasinating piece of technology.
In the new mustang alot of things were changed from previous versions.
One item is the "control" of the auto.
The shift pattern (when it shifts) and how fast (hard) it shifts are all tied to a computer and the throttle.
If you ease into the throttle the shift pattern will change and will also seem sluggish (Less hard). If you WOT you will shift sooner based on the rpm and torque and faster (hard).
This keeps the engine within its torque band, increasing acceleration performance. Hence the fast shifts from 1st thru 3rd when you jump it.
Do a little experimenting and you will notice the difference.
As for driving in a fixed gear it will not hurt the
tranny.
But be advised a continual high rpm will cause heat in the
tranny.
That is not good for any transmission.
If you use it for it's intended purpose (Decelleration, and the occasional drive thru the neighborhood at higher rpm's) you wont have a problem at all.