As far as I know, you can definitely use a pair of symfonisk as the surround of arc. If you go into the system page of the arc you should be able to see the option of add surround speakers, then you can just follow the steps.

Great. Thanks for the info. I’ve had a look and it does list them when I go into that setting. I’ve not ordered them just wanted some advice before buying. 
 

do you think two SLs would sound better or will it not matter for surrounds?

I use a pair of one sl because they look and feel better than the symfonisk products. However, I do not think the sound quality differs  very much. Plus, the symfonisk products can fit into narrower spaces so you can have more flexibility in where you place your surrounds.

Thank you so much for your help. 

I have symfonisk lamps as surrounds and they sounded exactly the same as the ones they replaced to my ears. Reviews online say they sound different than the ones but I didn't hear it. 

Personally I like the look of the lamp much better. Everything from the packaging to the materials of the lamp is on par with Sonos products imo. The shade of the lamp can be changed so you aren't stuck with the mushroom head look

I’ve just ordered so finger crossed :)

How did it go? Looking at doing the same.

Work great. Sound quality is spot on.

I also use 1 pair of Symfonisk with my ARC & no Sub, sound is good….