For each style there are 4 single touch setting pads. Each pad can be a single or layered sound and would do exactly what you want to do. You could have piano on sts1, strings on sts2, piano and flute on sts3, etc. Also you can have I believe it's 128 performances created and if you turn off the style light button right next to the performance button, you can now change to any of the performances without changing the style.
The 4 assignable pads are not to play sounds on the keyboard but rather to just play a single sound like a special effect, etc. each time you play the pad. These pads are not intended to be buttons to change your sound played on the keys, the STS buttons (4) do this.
George Kaye
Kaye's Music Scene