Please note that private networking on DigitalOcean doesn't limit this network interface to your own instances. All other customers on the same datacenter can connect to services exposed there. I don't have any experience with RethinkDB, so I might be wrong here, but you probably opened up your cluster to all other customers.
The `fleetctl` command is incredible. Just being able to say "give me six RethinkDB nodes" without having to ssh into six machines, or writing chef/puppet scripts feels like a small improvement, but it makes everything dramatically better in practice.