Is it possible to have 6 Sonos speakers in one system for home theatre setup?

No, you’ve already ‘maxed out’ the number of speakers that can be used as a ‘home theater’ room, except for the addition of an extra sub.

Sonos doesn’t make a device that can be assigned a ‘center speaker only’ role, so they don’t support separate Sonos speakers for front left and front right, either.

The best you can do is use those speakers in another Sonos ‘room’. 

@reking You say in your first post that you’ve put the 1’s (do you mean One’s btw?) in a new system. That was the wrong thing to do. You need to add them to the existing system, but in a different room or rooms, depending if you want one room with stereo-paired speakers, or music in 2 separate rooms. 
 

Personally, I’d use the Ones as the surrounds with the Playbase, and use the Play:5s for music. 

I agree with nik9669a, the 5s would be better as a stereo pair. I have an Arc and One SLs and they have no issues providing adequate rear channels.

You might even consider using the 5s in the same room as an independent Stereo Pair for music listening as you can place them for an optimal stereo image and might find you like that better than the all-in-one front speaker for music. You could always group all for non-critical listening situations too.

Thanks for the responses @Airgetlam, @Stanley_4 and @nik9669a makes sense what you are saying. I will use the Fives as separate speakers for music listening (placed on either side of the Playbase to mimic surrounds) and the One’s for rear surrounds. Appreciate the feedback! 

The bolded part doesn’t make any sense.  If your speakers are at the front of the room, nor a part of of your home theatre room, they are not surrounds in any fashion.  You absolutely could use them for music streaming sources only, but I would not recommend grouping them with your HT speakers when you do that.   You’re not really getting the advantage of having wider left and right channels since the playbase will be playing those audio channels as well.  The only reason you’d want to do this is if your playbase has a sub attached, and you want to take advantage of that.  However, I’d then question why bother with the separate Sonos room for music at all.

A lot of people do have the separate Sonos room for music only, and go without bass if they are happy with the bass level Fives put out, which is rather good.  Or, they go ahead and get a sub for the music only room as well.

When you setup a Sonos Home Theatre, like the one mentioned, (Arc, 2 Surrounds & Sub) - they usually all talk to each other over a fast ad-hoc 5Ghz wireless network and all will play in perfect lip-sync with the TV video on screen, playing surround sound audio etc.

If you then want to send the audio to other speakers around the Home, then that’s done ‘usually’ in stereo audio and is sent either wired, or wirelessly, either over the local WiFi network, or over an internal wireless network, called SonosNet - in each case where wireless is used, the HT will use its 2.4 GHz WiFi adapter. It’s a far more penetrating wireless signal than 5Ghz, which is great for sending the audio to (grouped) speakers that might be on the other-side of walls, or located on different floors, but to deal with such home networking conditions, the Sonos players involved ‘buffer’ the playing audio. The 2.4Ghz WiFi band is also not as fast as 5Ghz, but it’s benefit are it’s more robust when operating through walls etc.

The buffering involved, may cause a very slight delay of around 75ms, or less, to the playing audio, depending on network conditions …and for really poor conditions the buffer can be increased in the Sonos App. That means if you place speakers ‘grouped’ with the HT main setup, in the same physical room,  you might notice a very slight echo -  but if you place those speakers some distance away and in another room, then all will likely play fine and may appear to be in sync as you walk between rooms etc.

So the answer is ‘yes’ the TV audio can play on other Sonos speakers, or ‘rooms’, in your setup, in fact it can play on up to 32 speakers in total, but you would need rather good network conditions to do that.

I personally have no issues playing on more than a dozen speakers (up-to 5 or 6 Sonos rooms, as an example) and that’s with my ‘busy’ Home WiFi network.

I will also just add another further thing. The Sonos App has a number of tools/settings available not only to adjust buffer sizes, but there are also some slider-controls too, to help bring grouped playing speakers into perfect sync, but note when using such features you may find the audio is then slightly out of sync with the video on the TV screen, but I find these tools/features useful when listening to music and music videos playing from TV Apps - YouTube, Amazon Music videos etc.

Anyhow, I hope that information also assists to help ‘clear the mist’, a little.

Correct.  You would group a single One with the Home Theater room.