If one is writing about a work of fiction, one may describe the work, and use terms from the work in doing so. One might write, for example:
In Spaceman Sam the characters use "blasters" as weapons, while in Jack of the Galaxy "contragravity" is used to propel "aircars"
These are facts, and copyright may never be used to protect facts. Moreover, such uses as reviews and criticism are particularly likely to constitute fair use under US law, or another exception to copyright under the law of some other country, even if some otherwise protected content is quoted.
If something appeared in that sci-fi universe prior to me re-iterating it; does that mean it's fair game for me to use?
If the "something" that appeared earlier is an "idea" or a name, it is indeed fair game. This is true whether it is obscure or well-known. In any case describing the facts of the fiction, even in detail, is generally not an infringement.
If "re-iterating" something from an existing work in a new fiction involves an extensive quotation, or a detailed, point-by-point reuse of a fictional element, that might constitute infringement, unless fair use or another exception to copyright applies.
Does intellectual law protect very specific plot concepts?
No, it doesn't. A plot concept is an idea. In the US, 17 USV 103(b) reads:
(b) In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work.
Laws in other countries are similar on this point. This is sometimes known as the idea/expression distinction. This means that while the expression used to embody a concept is often protected by copyright, the concept itself never is.
Are you allowed to borrow names of different generic sci-fi inventions that aren't super-important to the story.
Yes. In fact even the names of fictional technological devices that are very important to a story may be, and often are, reused in different works by different authors. For example Ursula LeGuin used "ansible" as the name of a faster-then-light communicator, in fact an instantaneous communication device, in her "Hanish" series, particularly in The Left Hand of Darkness, The Dispossessed, and The Word for World is Forest. (She wrote that the term was derived from the word "answerable".) The term and concept were reused by Vernor Vinge in "The Blabber" and A Fire Upon the Deep, and by Elizabeth Moon in the "Sassanek" series and more centrally in the "Vatta's war" series starting with Trading in Danger.
Moreover, the names of fictional characters are not protected. An author might introduce characters with names borrowed from Tolkien, Heinlein, or other well-known (or obscure) fictional works. US Copyright Office circular #33 "Works not protected by Copyright" says that "names, titles, and short phrases" may not be protected by copyright, and will not be separably registered.
However, if a character has not only the name of a previous character, but a detailed, point-by-point similarity in distinctive elements to a character in a protected work, that may be enough to make the newer work a derivative work, and thus be infringing if permission is not obtained from the copyright holder on the source work. The same is true of the detailed reproduction of a distinctive setting from a source work. But a mere name and broad, general description is not enough tom make a later work derivative. The more details are reproduced, and the more distinctive and specific those details are, the more likely the newer work is to be found distinctive.