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You should use rails RESTful architecture on your client apps to expose apis (xml, json), and some library like Net::HTTP to call those apis from your server(main) app.
It sounds like Pow may be what your looking for. It allows you to configure URL's for stack and rails application via symbolic links. It fits well in the rails model and may allow you to build your independent RESTful applications in the manor specified. I know their are methods for having independent rails servers work together but based this is a very low overhead approach and may be the way to go at-least for testing and development purposes.