I've enjoyed reading through these ideas.
Nonlinear population growth makes sense, and I think it would go a long way to improving the tall vs. wide dynamic.
* make planets, as well as starbases, require Administration (wait! don't shoot! arrghhh!! not the spleen again!!)
I don't think this is a bad idea, given some other changes. If this were implemented, the base administrator number would need to be higher (obviously), but perhaps there could also be a capstone-like tech that provides administrators. I don't know that the current capstone tech dynamic allows for cumulative change, but I envision something like a mid-level capstone tech that can be researched as many times as desired adding a single administrator at a time. So, if a player wants another planet and there aren't any more administrators, some time/resources would have to be diverted in order to pursue additional expansion.
The discussion about recalculating science contributions brings to my mind an alternative possibility--perhaps certain specialized buildings (research or otherwise) could have a minimum planet population requirement. In the long run, this would even out as each faction grew... but an early focus on building a smaller number of planets while pursuing large-population improvements could give tall civs an early boost that wide civs wouldn't enjoy. If such a population-requirement were added to specialized improvements, then sending out colonists to expand could temporarily shut the improvement down or diminish its effectiveness...
One other thing that strikes me as odd in Gal Civ III is how easy it is never to have a budget deficit. Sure, production and research may have to slow down, but I can expand without any worry of overextending my resources. As already mentioned in this thread, the mechanics of Gal Civ II prevented excessive early-game expansion--and I think that's not such a bad thing.
And we don't yet know what the upcoming expansion will do to overhaul the game's system of economics.
But I think more strongly tying population to economic production in some way would be helpful. I'm not sure of all the dynamics involved in implementing this, but generally speaking, at small population levels (e.g., below 5), I think planets should be more of a liability rather than an asset.