@ lethargie: Unless you are lucky with planet generation, you probably need at least three colonies, maybe four or even five. Industrial Planning will probably be required unless again, lucky with planet generation. If you do not use cores, then you want skills, at least Colony Management 1 to get enough adminstrators for that all-important Industrial Planning. Without IP2, planets need +2 on at least ore and volatiles to meet demand. Those are stiff requirements. If player is willing to spend points on colony skills, might as well go all the way for maximum convenience. But if player is interested in maximum power, he can get all of the benefits of colony skills with alpha cores alone, although it may take some time before player finds enough alpha cores.
As for bribes... if inspections only occur less than once every year, then colonies will make more than the bribe. The biggest danger is missing the alert and have Hegemony steal your cores due to Comply by default. Sure, if you bribe expeditions on top of inspections, you bleed income too quickly. (Bribing expeditions alone bleeds too much income from no-core user.)
As for more colonies, you still get income from population alone. Nobody says you need to put industries on the alpha colonies (although redundancy is nice, despite divided income). Get enough extra alpha colonies, and you can make a nice chunk of change from population alone, if you have enough extra colonies (like a dozen or more).
If anything, industries on those extra planets will probably be Military Base for defense against expeditions. Maybe Heavy Industry to bloat production limit. Maybe other marginal stuff like Commerce and Tech-Mining.