Hang on, do the Xindi join the Federation in the future? I don't recall. Or is it the case that they just ally with the Federation to defeat the Sphere Builders?
To explain why we don't see more of various Federation races serving on ships in Trek (except what you might call token Bolians), I've often had the idea that you get ships where the majority of the crew is made up of a certain species (such as the Vulcans in Take Me Out to the Holosuite), and we of course see mostly humans. Or some races don't contribute to the Federation by being in Starfleet, but rather in other ways. For example, if the Mizarians joined the Federation, you wouldn't think many would join Starfleet.