You can stretch Rails a whole lot these days -- so much so that by the time Rails is the bottleneck, you can probably afford to hire primo engineers to refactor

You can stretch Rails a whole lot these days -- so much so that by the time Rails is the bottleneck, you can probably afford to hire primo engineers to refactor to better tools. I would however build the real-time chat system with Elixir and Phoenix. Just knock that out and not worry about chat performance for a very long time. So I would build the main app with Rails. And the chat system with Elixir.