What are you hoping to achieve?

The best home theatre setup you can have with Sonos gear is the Arc Ultra, 2xEra300’s for rear speakers (behind the listening position) and up to 2xSubs (one of which must be Gen 4). 

They can be arranged as a single room (Sonos’ way of referring to a bonded set of speakers) and will give excellent Atmos audio on suitable media. 
If you add more gear in the same physical space (a room) they need to be done as a second Sonos room.

Playing the home theatre setup and the second room speakers as a group will not work. The way Sonos achieves multi-room capability is to delay the audio start. In a home theatre setup this is minimised as much as possible to avoid lip-sync issues. Any rooms grouped to the video source will have the 70msec delay, so those speakers are out of time with the home theatre ones. 

dang I wrote out a whole thing that was quite detailed that just accidentally got deleted. I spent like 25 minutes writing it.

as a synopsis I disagree that the supposed make up is the ideal set up and I say that having the exact proposed set up except with the addition of two era 100s. You are correct it needs to be in a separate group but sometimes when I’m watching TV and it’s not on I know that something’s not right because the experience is not nearly as full…

well something happened where I decided to swap and put the era 300s in the front and vice versa and make the era 100 surround (Doing that took like seven hours) And the era of 300 as a normal pair. I was completely blown away
 

At first I was going to purchase another pair of era 300s but then I saw that the amp allowed for the addition of non-Sonos as well as seeing that beautiful floor speaker that very much seems substantial as all hell. More so than a pair of era 300 I’ll tell you. But you’re right that they belong in the rear and thus I’m gonna put the floor speakers to the left and the right of the television. — Any sly tips would be welcome especially; something that’s new in this newest variant of the amp that perhaps is not in the classic five year old post.

I’m amazed that you don’t find the lag to the grouped speakers intolerable. But if you’re happy, it’s a good system for you. 

To be very clear I have a pretty technical mind and so from doing the reading before I even set it up it became clear to me that you had to plug-in through ethernet any unit that is kind of out of line of site. So just plug in the two subs, (One in the back one in the front), And plug in the arc Ultra and there’s no lag at all.. (To do this just get an unmanaged switch). So I got an eight port unmanaged switch for like 40 bucks and then plugged one ethernet in and now I have seven ethernetS out… and it’s plug-in play there is zero software there is zero networking there is zero knowledge needed. Plug and play. Literally. And they’re so reliable that net gears versions Have a lifetime warranty

I personally would not bother wiring a ‘bonded’ Home Theatre setup that’s located in the same physical room of the Home, as the secondary Surround/Sub devices communicate over a fast 5Ghz ad-hoc wireless connection with the primary HT device - rather than routing the audio through a wired switch. A ‘direct’ (fast) wireless link will likely see less latency  - I don’t see the point using a wired link, unless something is affecting their communication, which is unlikely, even in most exceptionally large rooms… I doubt there would be a noticeable lag, or dropout, even with devices that are not in the ‘line of sight’ with each other.

The real problems anyhow usually occur with TV audio only, when a Home Theatre setup is ‘grouped’ (not bonded) to other Sonos ‘rooms’ that are located in the same physical room, as the TV stereo-only audio is ‘buffered’ and sent to the grouped players with a ~75ms delay in their audio output - introducing an echo effect between the playing HT bonded setup and the grouped rooms. 
I tend to really notice that echo, especially when playing Dolby/DTS/PCM 5.1/Atmos surround sound on the TV Home Theatre… the grouped rooms can become really echoey.

Only time I would ‘group’ rooms in that situation is for playing music only audio, where the ~75ms audio buffer is applied to all devices… or TV audio, but only if the ‘grouped rooms’ were located in a completely different physical room somewhere around the Home, where the delay between them and the Home Theatre setup, could not be heard, even in the distance.

Good Lord, I wish they took that stupid YouTube video down for being misinformation!

To the OP, if you group another pair of speakers with your Sonos HT room in order to achieve “front” speakers, the following will be happening: The grouped speakers aren’t playing the front Left/Right channels, they are playing all channels; surrounds, Atmos, fronts, and center, downmixed to stereo.  Which means the actual surround and Atmos signals are compromised, ruining any positional/directional queues and screwing up the Atmos effect.  In addition, the soundbar is still playing the front L/C/R, along with the downmix in the “front” Era 100’s, muddying up the front soundstage that is already compromised because sounds meant for the rear and above are mixed in with the sounds meant to come from the front.

Also, no matter how you adjust it, you are going to have a delay, or lip sync problems.  In short, it’s a mess, and not even close to how the soundtrack was supposed to sound. 

And PS - Wiring via Ethernet does NOT eliminate the buffer to grouped rooms for TV sources.  It is 75ms with or without the Ethernet connection. You ARE hearing a lag, whether you realize it or not. 

You have been informed of these facts before, and now you want to add two Sonos Amps to the mess you’ve created??!!  More speakers does not mean better sound!

Only the listener will know the “best” sound (for that listener) when they hear it.

While I don’t enjoy the result achieved after adding additional speakers to a SONOS surround Room, I can imagine a person who typically attends very large venues would appreciate the results after adding additional Rooms.

True, though we don’t have to encourage others to not have optimal sound as defined by Dolby and/or the director.  Some don’t know they are hearing a bad soundtrack until it is pointed out to them. 

Something not mentioned (or I missed it) is the Boost, you really don’t want to use that.

Check the WiFi specs on the Boost then on your other Sonos and your WiFi access point and you will usually find the Boost is using older, slower and less robust WiFi modes. It like the antique Bridge that came before it are both discontinued.