There are two different kinds of questions that could be considered to fall under the subject of population ethics: “What sorts of altruistic preferences do I have about the well-being of others?”, and “Given all the preferences of each individual, how should we compromise?”.

Instead of "population ethics", you might want to use "utilitarian aggregation" in this sentence. I'm pretty sure "population ethics" is never considered to cover the latter question, which besides "utilitarian aggregation" is also studied under "game theory" or perhaps "cooperative game theory" or "bargaining theory" to be more specific. (ETA: I wrote a post which links to some texts on cooperative game theory, which you may be interested in.)