Just some notes from my attempt at setting up Hydra.
No need for manual database creation and all that; just ensure that your PostgreSQL service is running (services.postgresql.enable = true;), and then enable the Hydra service (services.hydra.enable). The Hydra service will need a few more options to be set up, below is my configuration for it:
services.hydra = {
enable = true;
port = 3333;