Hi @tdesaules 

Welcome to the Sonos Community!

In the order that you asked:

1. It sounds like a large room - I would recommend having 6 mono speakers over having 3 pairs of stereo speakers. Stereo imaging can be tricky when you don’t know what direction people will be facing or if they are facing in different directions. Having 6 mono speakers will stop this from being an issue. You can always try both - it only needs an adjustment in the app to link stereo pairs together.
2. Yes. Group the speakers together in the Sonos app, tell the one in charge what to play and they will all play.
3. Yes - all Sonos players are Smart Home devices and each “room” will show up in the Google Assistant app (once accounts are linked). A stereo pair of Fives would register as one room with Google. Note that you need to use voice commands to a third-party device to control Sonos - no screen controls will be available - and you just add the room name to the end of the command e.g. “Play Queen in Kitchen”.

I hope this helps.

I think the question of whether to go with mono speakers or stereo pairs, as well as Ones vs Fives, depends a lot on the size and shape of the space as well as how you intend to use it.  If it’s one big square/rectangle space with no specific areas where you expect people to sit or congregate, then sure, go with mono.  If there are two/three distinct listening areas in the space, then I would use stereo pairs directed at those spaces.  If you have a single wall that makes sense to be the stage or focal point for audio to come from, then a pair of Fives might make more sense.

Thank you guys for the reply

I think I will try first with the Sonos One mono mode. It's an "big" room 12m x 5m : living room (kitchen and lounge). I have to equip other room too so I will start with some One and test it before buying five.

Anyway thank you : the big question was about the ability to control a group of One with Google Assistant / Spotify as a room (first time over Sonos, and I can't find video with setup like that)