Moving database (Mongo) to separate droplet.

Hi. I have a single Ubuntu droplet with Nodejs, nginx and mongodb running. I would like to move mongodb to seperate droplet to split the load. Please, give some advice regarding next questions:

1. To allow app server connection to DB server, I can achieve it by enabling private networking for both droplets?
2. After doing all manual setup (like in official guide: https://www.digitalocean.com/docs/networking/private-networking/quickstart/, should I add some sort of encryption to that connection? As I understand - no, because traffic between these droplets will only be visible to private network.
3. Can I hide my database server from the public/Internet? If yes, then how I will connect to it via SSH for setup and administration?
4. Should I use a DO Load Balancer, or I can just use nginx for that task as long as I have 1 application droplet?

Thanks in advance!