You are correct in that the Earth Alliance consists in the main of
humans. Aliens are generally not integrated into the system, except in
very low-level stuff. (In some bronze-tech worlds where humans have come
in and pulled an India/England relationship, you may have colonial
governors who are native aliens, but are basically puppets.) There's
more than just Mars, Earth and the Sol colonies; there are a number of
other worlds and systems out there into which Earth has made a dent.