You can reconfigure a Sonos system, so remove the Sub from one room and add it to the other. Decide for yourself how you want things. 

“Question: I am now adding SONOS ERA 300 (1 No) : Is it better to pair the SUB MINI with ERA 300 or just add ERA 300 as a normal speaker and retain the SUBMINI + GEN1 Pairing ?”

Just your personal preference. Remember if you'd groep the Era with the Beem (and Sun and surounds) to keep using the Sub for TV sounds, the Era and the Sub will lag 75ms behind the Beam. By the way, stereo speakers are "paired", in a surround set up speakers are “bonded”.

“Question: 2 By adding ERA 300 as a normal speaker - will ATMOS experience dilute ? or will it be better if paired with SUB MINI”

What would “adding" mean in this case? Would you “group” the Beam with the Era? Then I seem to have read Atmos will end on both speakers. Do you want to "bond” the Era with your present set up? That is not possible - Sonos allows only two surrounds so the surround “room” in the app has reached it's max size. If you'd unbond the present surrounds, you'd need two Era 300's for surrounds. Since your Beam is a Gen 1 it will not bring Atmos to the Era's though.

To clarify - Can I just add Sonos Era 3 as a new speaker, and will it provide ATMOS experience. ?

Again, what does “just add Sonos Era” to you? Adding it to your existing system is totally possible. It will form a new “room” in your system. Bonding it to your Beam “room” will not be possible as explained above.

And what source will you get Atmos from? Atmos from a your TV will not work because the Beam does not do Atmos and the Era cannot be connected to a TV. Music will be possible on the Era only if you use the right music service.

Clear to me now . Thanks a lot for your help