They're going to limit to the Galar dex just like they did for Alola.
They'll obviously patch in later after people have had time to experiment with the new stuff.
They don't want everyone going immediately to Landorus etc
It's what ruined X and Y
I really don't know if that's the same situation. They explicitly said the reason was in order to manage the increasingly huge number of Pokemon, and to ensure quality control and balance. There was no mention of adding them in later, just that Pokemon are only transferrable if they are found in the Galar region.