hold the phone, this is something much easier they it's being made out. first, every component needs to be rated then whatever voltage you're planning on running it, ie if you're running a pedal at +9V 16V rated caps are fine, if you're running a pedal at +18V you want a minimum of 24V caps, etc. if the voltage of the caps are lower then the voltage then they will explode, and i do mean explode, it's just a matter of when. you always want the caps to be rated higher, and not just by 1V or 2V to allow room for error in voltage since you may not get exactly the voltage planned.
the easiest way to handle the situation is to always use high voltage caps. for me none of my caps are rated below 50V, so i never have to think "is this cap rated high enough?" the trade off is the size of the cap, and want to get the smallest cap with the highest voltage.
groove, as for the klon layout, pin 8 of the 7660s is +9V input. so you don't need a higher rated cap, but like i said, i remove all risk by using high rated caps.