Haven't they said Stargirl with be crossing over with one or more of the Arrowverse shows this season?
Not that I'm aware of.
If so, we might as well just include it now.
No.
I think at this point pretty much anything involving DC superheroes that airs on The CW can automatically be considered part of the Arrowverse.
That's not how shared continuity works.
Supergirl crossed over before it left CBS.
Again, Supergirl was specifically developed to be an actual part of the Arrowverse; Barry Allen ending up on Earth38 didn't suddenly make it a part of the Arrowverse... because it already was.