Page 45 of the primer about multiclassing leaves one point a little vague. It is very clearly spelled out that you can't multiclass as different archetypes of spellcasters, but the list leaves a little confusion over the definition - primarily 1st level ranger, divine champion, and paladin. At 1st level you still aren't a spellcaster by definition, but they are shown on the table as being unable to multiclass with another archetype of caster.
So could a 1st level ranger (with no spellcasting) start advancing in sorcerer, and thus be forbidden any further advancement in the ranger class?
As of right now I think you can, but thought I should seek clarification
Oh, and just as a note, you should probably add eldritch knight and arcane tricksters to the arcane list in the table for clarity, though eldritch knight is mentioned in the text above
AKA Kavaris, awakened "Human" from the Hinterlands, psionic transmutation specialist, adventurer, and no one important