I've read the FAQ and the big thread on Diplomacy, but I want to clarify a few things.
First off, there is simply no way for a race to peacefully accede to your empire, right? If you want more than one race in your empire, you have to conquer them.
Second, no matter how friendly you are with another race, you basically can't share a system with them, right? Even if they want completely different kinds of planets than you.
Once you've got more than one race in your empire, how can you see the colony cost from the other race's point of view?
Also, a question about the Galactic Map somewhat relevant to the subject. I'm not sure if this a bug or something I'm just not understanding. I'm in contact with two NPR's at the moment. When I click the box in the System Details sidebar marked "Control Empire", there's a choose box with all three empires in it.
If I select one of the alien empires, their flag shows up on Sol system, and nowhere else. While if I click on my own empire, my flag doesn't show up anywhere. This seems odd to me, but perhaps there's something I'm not getting. I would have thought that box meant, 'systems controlled by empire X'.