About genderless breeding

It is not possible to get Surfing or Flying Pikachu in Generation 6 games, because Pokémon that know HM moves cannot pass through transfer systems.

Consequently, if someone is offering you such a Pikachu in X and Y, you can be pretty sure it's hacked - at least, until an event is run distributing it!

EDIT: Upon a little more investigation, even if a Surfing Pikachu event is released, breeding it will NOT give another Surfing Pikachu, because Pikachu is not compatible with the Surf HM and it therefore will not be passed down - only TMs that can be learned by the child (in this case, Pichu or Pikachu as the case may be) are passed.

The chance of an egg being produced is dependent on the Original trainer ID numbers and Species of the pokemon involved.

The day care Man can tell you how likely an egg is going to be generated after each set of 256 steps going on what he says.

Taken from Bulbapedia:

• Same species, Different Original Trainer - "The two seem to get along very well" - 70% (88% with oval charm)
• Same species, Same Original Trainer - "The two seem to get along" - 50% (80% with oval charm)
• Different species, Different Original Trainer - "The two seem to get along" - 50% (80% with oval charm)
• Different species, Same Original Trainer - "The two don't really seem to like each other very much." - 20% (40% with oval charm)
• Different Egg Groups, same gender, or Undiscovered group - The two prefer to play with other Pokémon more than with each other. - 0%

That's all there is to it. It didn't need to change as the generations went on, they've just added more mechanics into it, like IV and nature breeding.