Almost all of the API-focused projects use API endpoints for testing, so in theory, you should be able to still use Rails and get these to pass. You can build on any platform - GoMix is recommended, but our goal is to give campers as much flexibility as possible.
I'd be very interested in seeing how many of these tests you can get to pass using Rails. You could create issues for any tests that won't pass with Rails.
Again, our goal is ultimately make these projects implementation-agnostic.
Note that all of our nonprofit projects use Node, so it doesn't hurt to get some exposure to it, even if you're mostly using Rails day-to-day.