No. Sonos supports fronts in the form of Soundbars providing the front left, front centre and front right speakers, plus two rear surrounds and Sub. That’s your 5.1 setup. 

Thanks. Would there be a way to add the back speakers as part of the overall “room” in Sonos app even if not in surround? 

A “room” is just a label for a speaker/set of speakers. Some folk have a 5.1 home theatre setup and a stereo pair of speakers in the same room. One might by labelled “Lounge” and the other “Music” to differentiate. 
When you group “rooms” they will play in sync, except when the input is tv through a home theatre device. In this case, a 70msec delay will exist, which might manifest itself as an echo. Some say it’s not an issue, but others find it too distracting. 

Update: the rear speakers seem to be working now.  So you can have an ARC, plus two sets of external speakers in surround. I don't know what made it start working, but I did press pause and play a couple times on the amp that both sets of speakers are wired to.

It is not the number of non-Sonos speakers that matters. The Amp can only power one set of terminals, how many speakers you hook to them the Amp doesn’t know and it only sends out one amplified audio (L/R) signal.

For TV, you can have the Arc and Amp as two Sonos Rooms (not related to rooms in your home) and the Arc will play the front channels of whatever you are watching. You can Group the Amp to the Arc and the Amp will then play the same left and right channels the Arc is playing but with a slight delay.

or

You can have the Arc and Amp as one Sonos Room where the Arc will play the front channels and the Amp will play the rear channels.

For music you can also do either but you have the choice of full or ambient modes if the Arc and Amp are one Sonos Room. Full mirrors the front, ambient uses filtered versions.

All of this makes more sense if you think back to the original Sonos wording where Sonos Rooms were called Zones to distinguish them from rooms in your home.

Thanks Stanley for the clear explanation. That does seem to be exactly what I've got now. I'm curious whether the ambient versus full is only for music or can I also do that with the television? Since the speakers at the back are pretty far away, it would actually be nice to have it repeating what the arc is doing for football games or watching the news where surround is not really that useful.

Ambient/full is only for music. 
Since Sonos gear is so easily reconfigured, you could try setting up the Amp as a different room - Kitchen - and group it to the room with the TV. Then see whether the overlap zone - the bit where you hear both rooms - is in a part of the space where it doesn’t matter. If you only hear the sound from Amp when you’re in the kitchen, the echo won’t matter. If a seating area is in the echoing area, reconfigure it again, or try moving your speakers further into the kitchen. 

I’d look at adding a Era 100 or two to the kitchen area. You can listen to your TV with the slight delay, Group all for music sources or just play the 100(s) alone to keep the noise level in the house down.

We will rarely group our Kitchen One SL to our Arc when listening to shows where the dialog needs a bit of a boost in the kitchen area but when we do it helps.