I’m looking for and old science fiction title, probably from the 1970s. A small group of humans who are the last survivors of the human race are working in space as mercenaries and fighting for Aliens. In the novel they discover an old powerful Derelict space ship and are able to make it work.
Not sure if they are old enough but the Janissaries series from Jerry Pournelle sounds similar. I don't think they were the "last survivors of humanity" but they were carefully selected as "soldiers who were almost certain to die if the aliens hadn't rescued them".