Here are my experience based on your questions:
I personally use ExpressJS. I made a small project with Meteor a long while ago and it was very nice.
I don't use a template engine. I use AngularJS and then use a proxy like Nginx to route all API request to the Express server.
See above
In the company I work we use grunt, but personally I like gulp more.
I don't have experience with ES6 because I use TypeScript. A quick look at babel, I still can't say for sure if you have to use it but theoretically no because your server isn't consumed by browsers.
I think database also counts to the stack. There I use MongoDB and Redis.
I hope this helps a little bit ;) If not just ask - we don't bite :)