Yes..get surrounds. You’ll love it. I have the Ones and had the Era 300’s which didn’t impress me so I returned them and put the Ones back.

Shall I get Ones or Era 100? 
 

I am assuming Era 100 both drivers work when in surround? 

Only you can decide which ones. Easy to return either if you buy at a local store. I haven’t heard the 100’s. 

The Symfonisk speakers make great cheap surrounds too.

Would they really add clarity and quality when compared to era 100s? 

Probably not but at a 1/3rd the price they still add to the surround experience.  I don’t have an arc or beam 2 and find not a lot comes via the rear surrounds on Dolby 5.1 or 2 to make a set of era 100 worth the investment for us.  For most of our viewing the Rears surrounds do very little.  If you watch a lot of surround films your mileage may differ.

When era 100s are connected as surrounds do all drivers on Soundbar fire? What happens to side firing ones? 

Does Arc + Sub & Rears work as 5.1.2? Assuming Arc does 5 channels (front)?

Why would any speakers in a soundbar not be used when adding rear speakers? All speakers in your soundbar will keep working when adding rears. The Arc, if I remember correctly, does FL-Center-FR and upfiring speakers, so 3.2 of the 5.1.2. The other two are the surrounds the Arc isn’t tasked with and does not do when no surrounds are added.

Yes, i just got the one sl’s and they DO make the set up complete. I have the beam2, sub mini, 2 sl’s in a room size 4.5m x 3.5m and i cant have it on any louder than 30 on the app as it will be to loud on the surrounding  flats/neighbours. You hear better affects through the rears, ie. bullet zip, rain etc. Im glad i saved the money buying the one sl’s over the slightly more powerful era 100’s as they are perfect for my room size, if you have a room a fair bit bigger go for the era 100’s.

I have the One SL as a surrounds in an apartment and even those I feel are over kill. They never reach the speakers full potential unless you're forcing the surround volume up in settings.

If you can find a pair of used ONE SLs on Kijiji or FB market, get them.

Otherwise it would be perfectly fine just going to Ikea and using their Symphonisk speakers as rear surrounds.