Are any of the materials in Paizo's Starfinder rules considered standard content for Paizo's Pathfinder rules according to Paizo?
If yes, is any of that content also considered Pathfinder Society legal?
I will address you to this blog entry from Paizo, dating from 2017. What it says, in essence, is that while they tried to make most of the content compatible between the two systems, Starfinder and Pathfinder still work differently.
From the article, the cross content depends on the category:
Furthermore, the new books now come out with "Pathfinder / Starfinder compatible" tags. (Can't link appropriately right now to examples).
Most of the work is done to tell you what is compatible with the other system when you get new material.
I haven't been able to find a list of "approved cross system content", but given the information above, you can fairly reliably assume that a creature from Starfinder that is not in a Pathfinder Bestiary is the "official" and can be used with little effort of conversion in a Pathfinder game.
However, since there is (to the best extent of my knowledge) no sanctionned list of cross content, I'm afraid PFS legal is still restricted to current legal Pathfinder rules and does not include any extra content from Starfinder. The Societies are still there to make you play the system they're dedicated to.