Setting Music Playback to Full under the Surround Audio settings only affects music playback, not TV audio. But you would definitely benefit from using a pair of Fives rather than a pair of One SLs since you are essentially getting full stereo playback from the surrounds when streaming music. TV audio will be an improvement with the Fives too but not as noticeable.

The Arc cannot be used as a center channel speaker, and the Fives cannot be used as the left and right front channels in a Sonos home theater setup.

@Tegs 
The ‘Fives’ would sound better for ‘Full’ music audio playback, they are perhaps slight overkill for TV HT rear surround channel audio, but some with much larger rooms, seem to prefer them for that purpose too. 

Unfortunately the Arc cannot become the HT front center channel only, nor can two additional Sonos speakers be added as HT front surrounds, although some users in the community here have put forward that suggestion and I’m aware that Staff have mentioned they have forwarded on the idea, but nothing has been said about it. As you may perhaps know, Sonos very rarely, if ever, mention their development roadmap in advance, but I’m sure some here would like to see the option to add front surrounds too (me included) although my Wife says I need to keep some power sockets available to plug in the vacuum cleaner. 

I prefer FIVE’s in the rear. It’s debatable that FIVE’s will enhance the crickets in a forest scene, but I appreciate a more robust rear during chase scenes. If the choice is stepping up to FIVE’s in the rear or adding SUB, I highly recommend adding SUB.

Thanks for your answers,  its really the music I want to improve when streaming Spotify etc as I knew tv viewing would be marginal,  as long as I can use them as surrounds instead of a sterio pair but get much better music when set to full  id be happy ........ and is that basically the same as using them as a pair?

Yes, setting Music Playback to Full will give you full stereo playback from the surround speakers which is basically the same as setting up the speakers as a stereo pair, except you will also get full stereo playback from the Arc at the same time.

The only difference would be if you streamed Dolby Atmos music. With Atmos music, the surrounds will act as rear surround speakers rather than a stereo pair.