I think it's worth pointing out that it's not like the Alolan forms of Kanto Pokémon became like that over a matter of ten years either. The most recent one is either Rattata/Raticate or Meowth/Persian, both of which were brought over at least centuries ago. In the real world, that's not enough time for any substantial changes genetically, though it does sometimes create differences in behavior of these animals, or for allopatric speciation to kick in. (Dogs are a good example: Every breed is the same species, but pretty much every region that welcomed in these canines then bred them to serve particular purposes and gained very different appearances and temperaments as a result.)
And since contact with Alolans has been going on for hundreds of years, there's no reason why the Alolans wouldn't send some of their own Pokémon back with them to other lands (especially before they learned about invasive species). Just as people from Kanto brought over Meowths, perhaps the people of Alola reciprocated by sending back Rockruffs. And considering the Japanese eat sea cucumbers, perhaps they brought back a Pyukumuku or dozen and they became their own indigenous population.
(Certainly though, if we're getting an Australia region, I expect it to be overrun with either Bunearies or Bunnelbies.)