I wouldn't recommend running Nginx, Unicorn, Rails, Redis, and PostgreSQL all on one instance. Better to offload the databases onto their own VPS's.
Only those who have never experienced a corrupted backup or failed slaves think a database is something that is relatively trivial to manage.
You're much better off looking at platforms like RDS, MongoHQ, Cloudant etc.
Nowadays, in PostgreSQL is quite simple to replicate a db via log shipping[1]. You can even stream the WAL to an S3 bucket.
[1]: http://www.postgresql.org/docs/9.3/static/warm-standby.html
Making sure that master/slave scenario works when you need it to requires extensive testing and care. You also failed to address backups. I've yet to meet anyone who runs their own database who actually tests their backup.