Ask HN: What are the basics of Startup DevOps?

I'm the technical co-founder of a small startup. We have ~10 businesses using our product. Given the nature of our market and our current pipeline, there are deals that could add 100+ businesses to our system in a few days.

We're using Rails as the main framework. We have a web, worker, and cable hobby dyno. We have a Standard 0 Postgres database.

My question is what configuration and features of Heroku can I leverage to handle large jumps in load? How soon should I invest in New Relic or another APM? What other aspects of DevOps 101 am I missing?

My background is in software development more so than operations. Thank you for your help!