I have a 5$ droplet. This droplet is for development purposes. I want to create a graphql api with node.js on my mongodb database to be used in my mobile app. i am expecting around 200,000 users of this app
If you have an expectation of 200k users going in, I'd recommend separating NodeJS and MongoDB from the start -- that way you're not having to do it later on.
As for using containers -- it really depend...