You can. The two speakers are regarded as identical for these purposes.

But depending on your front speakers, the One/SL’s will be rear speakers rather than “side”. 

@John B and @nik9669a both gave you good answers.

Question: What are you planning to use as the front speaker as your profile does not indicate that you have the requisite Sonos equipment (i.e. Beam, Playbar, Playbase or Sonos Amp)?

But i was told sonos is only a 5 channel system so no matter what the "one's" would be the side speakers because anyone that knows anything about surround sound systems knows that true 5.1 doeasnt have "rear" speakers. Those are for a 7.1 system. There are side speakers, a center, a left and a right, and a sub for dolby digi  5.1.

Technically, Dolby Digital has “surround” speakers, per https://www.dolby.com/us/en/technologies/dolby-digital.html. You’re welcome to argue that surround speakers should be not behind your listening area. But I think you’ll find the majority of systems  and encoding practices assume that these “surround” speakers are in the rear, on either side of the listening area.  It’s when you move beyond Dolby Digital 5.1 at which you start getting “side” speakers. 

I called in the other day and spoke to Sonos technical support, and I explain my theater set up (Arc, Sub and two play ones for rears).  They told me that I could add a pair of One SL’s as side speakers with the theater system, but from what I am reading within these community threads is that that is not possible. Please help me clarify.

Thanks,

Roger

A surround system with Sonos is 5.1 only. The soundbar is FL-center-FR and the One (SL)’s are the rears. The Sub is the .1. There’s no room for extra speakers in this set up. Either your where  misinformed or you misunderstood.