If you mean for surrounds, Eras are not compatible with Playbase, so Ones would be the way to go. Sub Mini will also be fine.
If you mean to add as an additional room, the Eras can be grouped with your Playbase.
Can someone please be more specific about the Playbase not being compatible with the ERAs as I am just about to purchase a pair of ERAs to upgrade Surrounds.
As stated here:
https://support.sonos.com/en-us/article/surround-sound-guidelines-and-limitations
¹Era 100 can be used as surround speakers for an Arc, Amp, Beam (any generation), or Ray. Era 300 can be used with an Arc or Beam (Gen 2) only
Ones look to be out of stock and may or may not come back.
I’d grab a pair of refurbished One SLs from here. Soon.
https://www.sonos.com/en-us/shop/one-sl-b-stock-shadow?sonos=native