I would expect some sort of Confederacy structure. Each planet or moon with at least a certain minimum population (say a hundred thousand people or so) would have one or more representatives in the Senate, with underpopulated areas falling under the representative of the nearest represented planet. From what we see in WCIV, the Senate has one house instead of two like the United States, but it is not known if all represented worlds have an equal number of Senators or whether they are apportioned according to population. Legislative powers seem to be invested in the Senate, with Executive powers invested in the President (and presumably there is a Supreme Court as well). We know little of the Confederate constitution, but it is likely that planets have autonomy in domestic matters as long as their local governments remain democratic in nature--the Confederation presumably guarantees basic rights to its citizens. In all, I would liken the Confederate government to the American govenment between the spread of the telegraph and the outbreak of the Civil War--communications are pretty much instant via tachyon transmission between jump bouy relays, but physical transportation between worlds still takes weeks, so heavy centralization is extremely difficult.