So a goid question is, what is the federation government like? We attribute it to American republic system with a president, council, and maybe a legislative branch of some sort.
What if the counsel is like the UN ruling councel, and each "Spicies" gets 1 vote I.e. 1 human, 1 vulcan, etc. Regardless of amount of colonies etc. So 150 member soicies, 150 member Council.
Now could have a "house of representatives" type of lower body that has a represinative from every member world and colony, and if able protectorate representative. This way, each individual world would be represented in some way. And maybe the counsel members are confirmed by the house that represents each speces, ie all human worlds and colonies would vote for the 1 human council rep.
This way all worlds would be represented in the federation, not just the main human, vulcan etc. World's.