Prime Directive is set in the Star Fleet Universe. The following is a quote from the Prime Directive Source Book, 1st Edition, page 11:

Our Romulans fire the Original plasma torpedoes, are organized into powerful families, buy ships from the Klingons on occasion, and cloak whenever it suits them [though in a manner like Balance of Terror, not the total undetectability of TNG-Bryan]. Our Gorns eventually became Federation allies against the Romulans and our Tholians remain fiercely neutral at the juncture of the Klingon and Romulan Empires and the Federation itself. (When threatened with invasion late in the General War, our Tholians did become temporary Federation allies.) Our Orion Pirates (composed of both native Orions and renegades from across the galaxy) fly stealth ships (often with cloaked) fitted with whatever weapons they can buy or steal.

The Star Fleet Universe includes other races: the feline Kzintis (Federation allies), lynxoid Lyrans (Klingon allies), tripedal Hydrans (who breath, or perhaps eat, methane), unknowable Andromedans (from another galaxy), reclusive WYN’ (Kizinti, Lyran, and Orion refugees hiding in a radioactive star cluster,) the peace-loving Interstellar Concordium and their fascinating member races, and the Seltorians (incsect-like creatures from outside our galaxy).

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