“Pair” requires two identical speakers. A pair of ERA 300’s and a pair of PLAY:5’s will be two stereo Rooms on the SONOS controller. Rooms can be Grouped to play the same music, but this is not surround sound. SUB can be “Bonded” to a single speaker,  pair of speakers, or a surround system. Here are some supported combinations.

Note that when grouping era 300s with non atmos capable rooms, like a pair of 5s, the era 300s will only play stereo.  Basically, sound quality is lowest common denominator when grouping. 

@buzz 

In the link above I assume your intent was to provide combinations of how a Sub can be bonded to speakers? If so the link is broken as it directs me to “Listen to Dolby Atmos Music on Sonos”.

I couldn’t find an all inclusive link to answer this question. Here’s another piece of data if the intent is to create a surround system. And some more data on SUB combinations.