OP, As stated above, the pad you posted is not only acceptable, but required. I show AQHA up through the Congress level. You HAVE to use this pad. Obviously over fences you will need a shaped pad and wear you number on your back.
As far as half pads, I have used a handful of different combinations, depending on the horse. To reiterate what has already been said, using half pads will take up tree space on your saddle so exercise some caution. It depends on what you are trying to achieve. I always use some time of non-slip pad underneath as all my horses have very nice pronounced, deep set withers that tend to ride my saddle backwards throughout a long day. I have used a mild front riser on a downhill horse to get myself rocked back a little more. I have used a sheepskin/gel half pad (one that I ride over fences with and use nothing else with it) both underneath (which is a little tight and mildly uncomfortable looking) and on top. It does not have white sheepskin but rather a very light off white, and I’ve used it on top even at Congress. I have a friend who uses a similar off white pad on top. No idea if any of the judges are actually offended by it. If they’re offended by my off-white sheepskin, they damn sure better be offended by the pink pinstripe Oglvy pad that I saw last year.
I’ve also shown some just the number pad with a non-slip pad and nothing else.
Bottom line, make sure it’s not offensive to look at (black, eccentric colors, etc) and fits the purpose in which you want a half pad for.