technically, a mouse sander is just a small footprint orbital sander (funny shaped palm sander).. a RANDOM orbital sander is a different sanding pattern, similar to a DA....
belt sanders have no business anywhere near a car, leave that for sanding your dining room floor or similar...
little tip... put your mud on, let it flash off 10 minutes or so, than lightly go over it with 36 grit to just a bit over finish height... rips it down in nothing flat.. tho it'll load the paper up, it's 36 grit and can be tapped/blown clean easy enough, or just go thru a couple pieces... than finish it off when fully dry..
or go get a cheesegrater,
and use it in the same fashion.. couple bucks in the autobody supply isle.... same technique, scratch the bondo with the corner of it, when it's dry enough that it leaves a lighter colored scratch, file it with the cheesegrater to rough in the basic shape...