Sorry for the late reply. My bot is live for ore than a month now and is being used by a lot more than 100 users. Here are the answers to your questions:
I opted for ubuntu server with 15GB RAM(though i read that 2GB was sufficient) and memory of 250GB (again taking more than required).
Nopes, I believe the scaling is built-in in RASA. Even my front-end is on FLASK and i am able to handle multiple users along with different sessions for each user without any explicit scaling.