Let's start with few questions that can lead us to better answer.
Ultimately, how many users are supposed to use this app. Is that for internal use among some company workers? How many chatrooms are planned? I know you consider scaling, but this would still be useful to be addressed.
Will the messages ever be saved to the database,or only passed further to all available clients at the moment? Will users or admins have some access to chat history, will there be some way to mark messages as unappropriate (spam, abuse, other potentially unwanted stuff).
That said, by default, Meteor's pub/sub system is not the best option for large scale chatting applications. This may be better done with some simple system like Socket.io or Deepstream, unless you use some customized system to send messages with Meteor, f.e. as streams. New Redis Oplog comes pretty handy with that, but as it's quite new, there's not enough data yet to prove it at large scale.
If your main reason to choose Meteor is because of the authentication system available out of the box, I'd reconsider it. The authentication in Meteor is awesome, I admit, but that shouldn't be your main worry.