On the subject of PSO2, given this is now the official thread for it. The year before I asked a question about PSO2 that never got answered.
Aw.. To think that I missed the post.
For that I've read in Idola Phantasy Star Saga Discord Discussion :
PS Tetralogy, PSO, PSO2, PS Universe, PS Zero, PS Nova, and Idola ...
They are not in the same dimension/universe, meaning that they are separate continuities.
For example: Earth is already destroyed in PS II but still exist in PSO2.
But they share the same enemy : Dark Falz / Dark Force,
and they can go into other dimension using the Star Gate.
That's why Matoi, Phuramy, and Quna from PSO2 could show up to in Idola Universe,
via the Star Gate.