One thing for certain is that sectors are bigger than quadrants in B5.
Recall Quadrant 37 which was cleansed of Narn at Londo's request by the Shadows:
Quadrant 37 was system on the edge of Narn-Centauri space, located in Sector 157.
I think you are wrong that "specific planets" are referred to as quadrants. Quadrant 37 contained at least one planet, but was not the name or designation for the planet.
Also, sectors are fairly small considering the following:
Sector 21: Sh'lassen Sector. Known to be strategically close to both Narn and Centauri space.
If Sector 21 is between Narn and Centauri space and both empires fought over parts of Sector 157 too, it suggests that JMS' galaxy is divided very finely into sectors and even more finely into quadrants.
Other than this, the exact meanings and exact relationship between sectors and quadrants were not spelled out in B5.