Yes, you can use the old clip.
Make sure you fit the new boot on first...yes, I've fitted a few CV joints and forgotten
Squash the clip to make it more round by pressing in the open ends of the clip with mole grips then slide the clip back onto the driveshaft...this will make it easier to fit the CV joint back on...if required you can press in the open ends of the clip in as you push the CV joint back on, but squashing the clip will be fine.
If you don't have a plastic or copper hammer then screw the big nut back onto the shaft so you don't damage the CV joint threads when you knock the CV joint back on.
There is a photo of the ball joint protector in here.....
viewtopic.php?f=10&t=20950
It isn't vital to have it fitted.