"Forward slip" and "side slip" not "forward side slip" ... two different things, although the same
I've never heard of rudder overbalence during a side or forward slip in a light aircraft ... but I may be wrong?
It's amazing how these sort of threads always come back to what or what should not be taught at PPL level. Or if one kind of skill is relevent to any other kind of flying. Does it really matter? You simply can't teach everything at an early stage (although side slipping would be worth it "oops, I'm at it too"). The point IS that side slipping is a VERY useful skill to know, takes little to learn, isn't that hard to perfect and could ... just could save you or your aeroplane during a forced landing. It's also good fun ... GO ON ... anyone who can't ... learn to do it next time you're up with an instructor
(and if he/she doesn't know how to either at their stage in the game ... change instructor!!!)
SS