I think the only real hint at naval ranks in the Rim Worlds is the Reunification War book. There we have a Admiral Hakim Wibka who led the small Rim worlds fleet.
And it seems as if only the Rim Worlds had seperate ranknames for their navy officers as for example the Taurians had Marshals leading navies like Marshal Cardenas or Marshal Santos while Marshal is also the rank for leading ground forces.