Virtual surround

Hi

What speakers do you currently use as your post implies you already have a stereo pair of some brand or something upfront. Know that Sonos for music will only integrate with non-Sonos speakers that are connected to an 3rd party Amp via a Sonos Port connected to the amp with RCA or Optical input and/or RCA output. Even so if Sonos speakers and the 3rd party speakers are in the same room in addition to a 75ms delay you may (?) also have volume equalization issues when sending streamed audio to the 3rd party speakers via Sonos as the source.

Note:

If your current speaker or speakers are Sonos Play 1’s disregard all the above and purchase an Era 100 x 1 or 2. The speakers will integrate seamlessly.

I am using a full Sonos front as a home theater.

with 2 x 5’s and a sound bar Beam and sub all in front 

I want a small speaker as a filler at my back.  Hopefully a Sonos as well to connect easily.

What can you recommend?  Thanks so much!  

There’s no real way of doing it if your input is tv, as there will be a delay to the sound sent to your “filler” speaker. 
How are your speakers currently configured? Do you have the Fives set up as surrounds (in which case they should by convention be behind your viewing position) or are they a second room, grouped with the Beam?

A pair of 100s or older Ones or Play 1s would work as surrounds.

You can only have one pair as surrounds so you’d need to remove your 5s from the home theater set to add the new ones.

You’d then have the 5s as a different room that could be grouped or used alone to play music. Not so good for TV due to the delay issues.

Just to add my own experiences of grouping other rooms with the home theater room if source is TV. 
The echo/delay can be eliminated quite good using the „tv dialogue synchronization“ option. Of course technically this will cause a minimal delay between audio and video, but imho it’s not noticeable as a handicap. I think it should be mentioned as an option that every user can try and decide on his own if he’s happy with it. 
Setting the option to level 3 for me works perfectly. No audio delay between the grouped rooms and no remarkable video delay. 
I just use that sometimes to group tv audio to the dining room that is part of a big open living/ dining room. 

No offense, but if you are using surrounds as fronts, it's not really possible to make it sound worse, as you've already screwed up any front soundstage and surround effects.  Group in as many speakers as you like, you're already at rock bottom.