Sonos five (stereo pair) with Sonos Era 100 or Sonos Era 300

Hey! I actually have both. I moved the 100s into a smaller room that has a tv, and the 300s sound fantastic in the living/tv room. I also have a 5 in there as well. Your room is slightly bigger, but it should fill the room nicely. They are very nice and plenty loud. I have two subs as well in that room. I would make sure you have a little space behind your seating to accommodate the spacial atmos. That’s my only issue, my couch is close to the wall, but any space behind your seating should be enough. Hope this helps

The Fives Grouped with the 100s are going to sound very good, less maximum volume than the 300s but that is likely good enough for a secondary set of speakers in a room. If you don't need voice control the SL 100s will save money.

The 300s would be a bit louder but when Grouped with the Fives won't do Atmos, only stereo. The extra ecpense for two more Fives instead might be a better choice.

Edited to add:

Maybe an Amp and some big floor standing speakers if you really need the volume. 

Also if it is just the low end that is weak consider a Sub as an option and stay with two Fives.

Experiment with vertical and horizontal orientation of the FIVE’s. SONOS deliberately changes the imaging based on orientation.

In this case, is it possible to use ERA100 stereo pair speakers as rear speakers?

Accordingly, before that, by separating Sonos five stereo piar and Era 100 stereo pair speakers for different rooms using the app, and then place them in the same room. Will there be a loss of stereo sound?

A stereo system consists of two speakers only two,  multiple sources are no longer stereo, but “home cinema”
 

Thank you all for the reply.

I think it will help me in my experience.

Surely a “home cinema” system is one playing 5.1/7.1 or other cinematic audio signals, rather than multiple speakers playing a stereo source? A room with multiple speakers (single mono or speakers or stereo-pairs) playing the same audio source is simply a multi-speaker system - and not necessarily producing a good, cohesive sound. 

Placing a pair of SONOS five speakers 2-2.5 meters apart will provide optimal stereo sound. Further distance will degrade the stereo effect. Since the room is large, the stereo effect will be weakened. To reduce the loss of stereo sound, I planned to add a stereo pair of ERA 100/300 speakers (as rear stereo speakers, 3-4 meters apart, positioned closer to the listening position and directions for the listener). These speakers would operate separately (configured as a separate room), but in the same room as the SONOS five.

Home theater systems operate in 5.1 or 7.1 surround sound, but in my case, I wanted to preserve stereo sound rather than create surround sound. I'm looking for quality, not volume.
The question is, will the SONOS five and the ERA 100/300 stereo pair work in sync when listening to the same source, or will distortion (echo, latency, etc.)?

There's no such thing as "rear stereo speakers", but I understand what you are trying to do: have four speakers play music, so stereo sound from speakers in front and speakers front and behind you. Since you are planning on grouping two "rooms” in the Sonos app, why the insistence on Era 300's? An extra pair of Five's will fit sonically with the Five's you already own and the Era 300's USP (Atmos) will not work when they are grouped with the Fives..