Add these benifits
I suggest that you have a rewards system that is based on the level of each race, to promote playing all the races.
For example, once you have all Sub-lieutenants for each race you get star-base money transfer, or starbase ION shield for all races or bonus resources for each race. Rewards each time you complete a level of rank across all races.
Summary:
all races Sub-lieutenant + 100 of each mineral for each race
all races lieutenant + 150 of each mineral for each race
all races Commander + 200 of each mineral for each race
etc.
Better yet would be a reward that can only be achieved via this action. I.E. Natives breeding - ability to take a portion of the natives from a planet and place them on another planet, or Amorphous taxation -ability to tax the amorphous. OR amorphous farming - ability to have the amorphous to act like bovinoid.