I have an Aussie saddle (which I love) with a felt underside to conform to the horse's back. I use a really thin pad with it, like a dressage pad. My horse hasn't complained and I haven't found any soreness on him, but is it usual to use so thin a pad?
I have always used thicker pads with aussie saddles (Im assuming you mean a stock saddle?) but if the saddle fits correctly then there will be no issue with a thinner pad. Besides perhaps sweat getting through the pad and damaging the felt underside over time with sweat.
Yep, you WANT to use a thin pad with an Aussie saddle. Like an English saddle, it has the padding actually attached to the saddle (a western saddle does not = why western pads can be so thick). If the saddle fits well, all the pad is meant to do is keep the saddle sweat/grime free.
If the saddle does not fit well, some fit issues can be helped via a thicker pad but if the fit issues involve it being too narrow, a thick pad will actually make that worse. Similarly if you pad up a really well fitting saddle, it ends up being like if you wore thick socks with well fitting shoes - sorta ouchy after a few hours!