In terms of the aliens' publication dates, A2 Research Station Gamma (1980) appears to be the first mention of the Six Races, with the following text in Library Data:
"Major Race. An arbitrary distinction based on the achievement of a specific
intelligent race. A race which achieves interstellar travel (jump drive) through its
own efforts is classed major; one which does not is classed minor.
To date, generally accepted major races include Humaniti (Zhodani, Solomani,
and Vilani, but not other examples), Aslan, the Hive, Centaurs, Vargr, Ancients,
and Droyne.
The Droyne have been demonstrated to be major, having developed jump drive
as far back as - 7000. At present, however, they do not build or use jump drives,
and remain voluntarily sequestered in their scattered systems to spinward."