adding speakers for additional surround or room to room audio.

You can only add a second Sub to your home theatre setup. 
The volume in each room should drown out anything from the other room. 

I have home theater rooms and music rooms, some in the same open-plan space and there is no system conflict with the multiple Rooms.

For music they are easy to Group together to fill the big space with sound.

For TV there is enough delay to be noticeable on the more distant additional speakers, the closer ones have enough impact on the HT Room I don't often Group them for TV. My other Rooms are separate enough the delay isn't noticable.

A single speaker, a “Bonded” pair, or a Bonded surround system is each known as a “Room”. Rooms can play independently or be “Grouped” to play the same music. You can also Group with a surround Room, however, TV audio in Grouped Rooms will be delayed slightly. This will be an issue if you are attempting to use this audio with another TV screen or if you are able to hear both areas.

I want to add speakers for music only, so that when I move from room to room I can experience the sound without interruption. My main listening will be in one room.

Do I need to add a pair to each room?  I’m hoping to add an Era 100 to each of two rooms or if that’s not ideal I’d add a pair of 100’s to one room now and a pair to another later. Should I hold out and add Era 300’s?

As a side note, I see that Sonos is offering me 25% off my next purchase. I’m assuming that is direct from Sonos and not through a retailer. I tried a test run order, but I didn’t see the 25% off credit. I didn’t want to enter my CC number without seeing the credit. I suppose the best is to call them during business hours to be sure. 

Thoughts?

Your current home theatre setup will also play music. Just set up your new speakers as a new room, and then group the two rooms to share the music playing. As long as you’re not using the tv as the music source - ie set up a free service like RadioPlayer - and your music will be in sync so you will hear seamless music as you walk from room to room. 
The 25% should be added (deducted?) at checkout. 

You can start with a single ERA 100, ERA 300, or a FIVE, in each of your secondary areas and add a second speaker, perhaps as a stereo pair, in one or more secondary area later. Only identical speakers can be Bonded as a stereo pair. 

Bonding or un-Bonding is easy -- just a click in the controller.

There’s no need to use identical speakers in each of the secondary areas. For example, one or a pair of ERA 100’s us usually sufficient in a a bathroom, but you may want larger speakers in the larger areas. If you want more bass you can add a SUB.

Any combination of Rooms can be Grouped to play the same music and the music will be synchronized throughout the Group.

Another possibility is to use a portable speaker or pair that can be taken outside on occasion. Note that you cannot add a SUB to a portable.

I really like the era 300s but the cost is a little bit high. I suppose if I want good music I need to pay a good price. Does anyone have any input on a pair of era 100s over a pair of era 300s for an auxiliary room?

At the moment, my auxiliary room is very small (10x8) however, I would rather overpower the room and plan for a larger space in the future than underpower the room and regret it later.

Sonos systems are easy to reconfigure.
 

Use the “remove surrounds” feature to free them up in your system. Move them into the auxiliary room and set them up as a stereo pair. (Do the same with the Sub to see what it adds to the Eras.) See if they overpower the room. Get some music playing and try the grouping feature too, to have the two-room musical experience. When you’re ready, separate them from being a stereo pair before reverting to the surrounds setup. 

I'd recommend a Pair of the Era-300s to start, try them in your rooms as singles and as a Pair and see what you like.

I love my 300 Pairs, big step up from the 100s, that I also have, and we'll worth it to me.  Particularly as I get ATMOS music from Amazon and am amazed at the sound of properly mastered tracks.