I don't think they're synonyms, but I also don't think interspecies-diplomacy needs its own tag. inter-species maybe, then you could pair it with diplomacy when you're specifically talking about governments.
Google definition of diplomacy: the profession, activity, or skill of managing international relations, typically by a country's representatives abroad.
I don't really think (for example) Culture shock… Alien Social Activities should count as a question about diplomacy, but it certainly has a lot to do with interspecies interactions.
I guess what I'm saying is that we keep diplomacy, but change interspecies-diplomacy to just inter-species. Not sure how this is done, but I think it would help.