Are there any naming convention guidelines for REST APIs? [closed]

I think you should avoid camel caps. The norm is to use lower case letters. I would also avoid underscores and use dashes instead

So your URL should look like this (ignoring the design issues as you requested :-))

api.service.com/hello-world/user-id/x

The REST API for Dropbox, Twitter, Google Web Services and Facebook all uses underscores.