Hey,
I found a tutorial that helped me figure out the rest. I don't know if this is the best way but it worked for me so here it is:
1. Go to your combi -> Menu -> C-FX Routing -> Bus
2. Select IFX1, IFX2, IFXn... to each of your timbres:
3. Go to IFX and enable each of your IFXs and select Stereo Compressor:
4. Go to C-IFX, scroll down and set the following values for the instrument you want muted until you press the pedal. In my case, they are the strings (IFX2):
a. Out level: 0
b. Src: Switch pedal #82
c. Amt: +35
5. Go to the IFX1 (the instrument your want to hear when the pedal is not pressed, default) in my case the piano,
a. Out level: 35
b. Src: Switch pedal #82
c. Amt: -35
6. Then go to Global config (Shift + SEQ) -> G-Input/Ctrl -> Foot and select Foot Switch (CC #82) in the Foot SW Assign line.
And there you go. Alternatively you can do this if you don't own a pedal by modifying 4.b and 5.b by setting SW1 or SW2 as a src.
I took the screenshots from the video tutorial I mentioned which is the following:
https://www.youtube.com/watch?v=xeEEpG0PZFY
Hope it helps, later!