You can only add two surround speakers to a Sonos home theater setup. If you want to add the two Era 100s as surrounds, you would have to remove the Amp/in-ceiling speakers as surrounds first.

If you group the Era 100s with the Beam setup, you will likely experience a slight audio delay from the Era 100s when playing TV audio. Streaming music will be in sync.

I’d add the Era 100s as surrounds, removing the Amp, but keep it as a separate Sonos Room so you could Group all of them for playing music.

Get the 100s into an optimal position and you will be quite happy with them. Even getting them close will likely be an improvement.