So long as your character is coming from a region in which that specific Pokemon originates. To use your example, if your character is coming to Hisui from their native home region of Alola, and want to bring their Rockruff with them, then wonderful! Rockruff is small, able to be held in their arms, or be put on a leash. The same could not be said for say, Drampa who is a ginormous dragon that wouldn't likely be tamed, nor easy to travel with personally on say like a boat. Drampa would unfortunately have to stay behind

Many first and second stage Pokemon tend to be small and easy to travel with, so those Pokemon in mind will be welcomed to Hisui. Pokemon from Kanto, Johto, and Hoenn are all present in the original Sinnoh dex to begin with, so it's reasonable to conclude that those species eventually migrated- or were brought- to Hisui in order to be around in present day Sinnoh.

Other exceptions from the Hisui National Dex will also include Pokemon that naturally available in that time period, like the fossil Pokemon or Pokemon based on man made objects (we're just going to ignore that lone Magnezone lol). These Pokemon will also be available in our rare space-time distortion events, but until then, you'll just have to make do with what's readily available to you from the start.