As hilarious as it would be to make someone insta-lose by Skill Swapping or Soaking them for a DQ, Ruko is right. Basically, if the Normal-type move would have been under the effect of the ability (and thus would have had STAB) when the user selected it, it's OK. If not, it's DQ, since you've then deliberately used a non-STAB move.
As for the Megas rule, the only factors really are ease of enforcement and ease of understanding for everyone involved. If the masses prefer the Megas to be exempt from the original type only rule, it'll go through. I should also point out that Soak is also covered by the original typing rule as well, so Soaking an opponent with no Water-type moves on their set won't force them to switch out or lose.