True, I agree that when the game gets that developed the better way to run such a coalition is to simply tax, probably based on property/territory, everyone inside of your coalition and then pay a security corp/mercenaries to protect that area. As the game gets more developed, like in EVE people who want to protect areas of space and maintaining their power/the status quo will form large alliances out of smaller powers and individuals to protect their sections of space. That is usually how such "federations" work is everyone involved pays in a set percentage or price, then the entire area of space is protected from outside forces. Then your biggest threat will be politics and subterfuge.
At the end of the day though, if you want to be alone in an ever expanding universe, you will have to always live on the frontier, and the frontiers of today are the city centers of tomorrow. You can either go out and use your position to your advantage, welcome growth and attempt to take advantage, or every time it becomes possible for you to move out further, that is what you will have to do. It will be really hard to live alone in this universe, just like it is in reality except as technology and infrastructure progresses Dual Universe will feature a much greater degree of interconnection.
Like I said before, your best bet for a permanent, relatively secure home where you can be left alone, is the outskirts of a major power's territory. Join the Empire, or something like that, where they will attempt to keep a large amount of buffer space around their centers of power. You can live in the outskirts of this space for relatively low cost, you will probably be relatively restricted in terms of how much expansion you can do, and you will still be subject to their government, but you will be secure which will be really important in such an open ended game.