Part of the reason the Allied Worlds is so successful is the arrangement of the planets with respect to one another.
Welkinstead is a hot Neptune. Yih and Newhome are Welkinstead trojans. Sweetwater is at the 3rd Lagrange point of the Focus-Welkinstead system.
The planets of the alliance are at fixed distances from one another, making logistics easier. They're also a celestial extension of a "wide" empire. The theory goes that empires that are "wide", bigger from east to west compared to north to south, are more stable because they consist of a similar range of climates, resulting in similar cultures that are easier to keep together. "Tall" empires, that are bigger north to south, span a wider range of climates, have a less cohesive range of cultures, and are harder to keep together.
Similarly, the four planets in the alliance are all the same distance from Focus, receive the same amount of sunlight, and have the same year length. The two non life-bearing terrestrial planets, Newhome and Sweetwater, can be terraformed using similar techniques.
