Not a suggestion, but here's what I do.
I leave the foam inserts out on the work bench for a few days and if the foam goes back to near its original striped shape, it is reusable in my book. If there are any tears or parts missing, it can affect how true (round) your tires are and ultimately affect your performance. So I usually toss those.
For bashing and just having fun, if you didn't cut or tear the foam during the removal chances are you can reuse them. If your wheels are mashed up, I can't say for sure if you can reuse them or not. I don't usually crash my cars bad enough to crack or curb the lips so I reuse most if not all of my m-chassis wheels a few times. Typically 4-5 times.
As for firmness, it's like how you would change the psi on your real car to adjust to your driving condition. On my m-chassis they all run the Tamiya firm inserts up front and soft inserts in the back as I want firm sidewalls up front, yet forgiving sidewalls for the rear. Depending on different surfaces and temperature (if out doors) I would bring different sets of tires with different inserts inside.
But for my spec-racing scheme I have going, my cars all run the same 60D radial s-grip tires, inserts, motor, kit ESC, same body to enjoy how each of the chassis perform on the same and even playing field.