Help me choose the right database for a real-time app.

I'm embarking on a fairly large project, using Angular 4 with Express, to create a live "debate" site where 2 debaters can use a realtime chat to debate, while spectators can use a spectator-based chat to participate and rate the debate as it happens. There will also be debate catogories (politics, religion, sports, etc..) that users can browse current debates / start their own.

There's going to be a strong element of live interaction for this app. Think of it like discordapp.com, except for debating.

Backend/db's are not my strength, so I'm looking for suggestions on which db's to consider for this app. Scalability is important, in case it gets a big mention and grows like I hope.

I'm thinking of using Firebase, as it handles authentication nicely. Does using that alone sound like a fit for this project? Outside of the fact that it does cost $ after a certain usage threshold, are there any other considerations? I understand the basics of Firebase, but I've never used it to structure a database that required different relationships and such.

Thanks guys.