Three Series

In “Hunters of the Sky Cave,” Flandry reveals that the Empire he has defended in “Tiger by the Tail,” “Honorable Enemies,” “The Warriors from Nowhere” and “The Game of Glory” is the same Empire that the leader of a slave rebellion, Manuel Argos, had founded in “The Star Plunderer.” In “A Plague of Masters,” Flandry reveals that Manuel’s Empire was preceded by the Polesotechnic League of Nicholas van Rijn’s period as described in “Margin of Profit” and The Man Who Counts. Since mercantile expansion was later followed by imperial decline, the combined series is about social change.
-copied from here.

This shows us that not two but three series came together to form the Technic History. "The Star Plunderer" was already part of a series because it had a sequel, "Sargasso of Lost Starships." Both refer to the secondary drive, not the hyperdrive, and "Sargasso..." tells us that the Empire proclaimed, then founded, by Manuel is nearly complete under the rule of his grandson, Manuel II.

The Ythrians were not a fourth independent series because their first story refers to planets introduced in the Polesotechnic League series.

"Sargasso..." describes the Shalmuans as originally aggressive which does not fit with what information we have about them but we remember that "Sargasso..." is a partly fictional narrative.