Not necessarily. If Fardy starts at 6, he can also play lock if needs be, if you had someone like Cottrell or McMahon (just examples) also on bench. A backup 7 is probably superfluous if Hooper plays.Skelton needs to be able to prove he can play 80 at Super level because if he's the lock cover on the bench and one of Horwill or Simmons goes down within the first minute or 10 minutes of the game, he's going to become a liability after 40-60 minutes.
If he can't consistently play 80 minutes of Super Rugby intensity, it is not going to get any easier in international rugby, where the intensity is another step up.
Anyway, Skelton could be carried on the bench by some permutation that would not be too outlandish.
Whether that is the mix / balance McKenzie thinks we need is another debate.