Just set size of generation region smaller.

thanks for trying it, i never did. did you observe resources both on the main island and the other islands?
i expected that new islands wouldn't have anything with RSO, only the main island

Resources (and biters) generated by default map generator has been wiped by RSO and new sparse ones created.
I can see process in sandbox mode.
Rampant and NE are present and works normal.
Xander-v1 resources are present too.

My new islands just got created, but are all empty of resources. Does it take time for RSO to generate them?

i don't know, i wouldn't expect it to take time. However i never tried this mod with RSO together.
I didn't expect them to be compatible and was surprised that McMaster mentioned it working.

another option might be to use
/c remote.call("RSO", "regenerate")
to regenerate the ores that RSO should have placed (this also renews ones that you maybe already mined away, so it's not ideal either)

Hmm, I'll still give that a try. I really could go for islands with RSO. The islands thing really gives a bit of flavor instead of all land.