Is κοινοβούλιο only used for foreign parliaments? I know that the Greek parliament is referred to as the Βούλη, but would that be called a species of κοινοβούλιο.
Both are used. They are practically synonyms for that matter. Πάμε στη βουλή=Let's go to the parliament (building). Το ελληνικό κοινοβούλιο αποφάσισε ότι...=The Greek parliament decided that.... The official title of the Greek Parliament is "Η Βουλή των Ελλήνων" meaning "The will of the Greek people" (βουλή=will, decision), whereas Κοινοβούλιο comes from κοινός(public)+βουλή(will)+ending -ιο. Βουλή is used mostly for the Greek Parliament, but you can refer to the greek parliament as το ελληνικό κοινοβούλιο.
So, could you say that βουλή would refer to the building and κοινοβούλιο to the "humans" (although they are synonyms and you can interchange them)?
It's true that Βουλή is the word that is used for the building (and not κοινοβούλιο), but is also used for the people and the official title of the greek parliament is Η Βουλή των Ελλήνων. So I would say that both βουλή & κοινοβούλιο are used for the people, but Βουλή is also used for the building.