I managed to figure out my own problem, and I feel I'm dumb. It turns out that the Trash contained lots of files and that prevented my system from being able to even start partitioning the hard drive. And I ask, is the message displayed in my previous post indicative of such a thing? Apple, what's wrong with you guys?
But wait, those are not the good news. If you try to partition a Fusion Drive at this point in time running version 10.8.3, you will end up with an incorrect sized partition. Apple, what's wrong with you guys?
... and should you attempt to modify the size, the GUI is hopeless. After a lot of work, I managed to partition the drive just fine following this http://forums.macrumors.com/showthread.php?t=1500361&page=2
Apple Support was clueless and insisted I should format my computer. I'm a little disappointed with Apple Support because you would expect you would be assisted by a real expert, but all they do is ask you to reformat your computer as soon as they can, when simply the Trash may be the culprit... And of course, they were clueless with the partition size thing, and even though they provided that link to macrumors, they wouldn't certify that information was accurate, nor would they even confirm that by reinstalling and reformating my computer, my problem would be solved... AppleCare these days seems to be useful for hardware stuff only, but phone support has become pretty limited/useless (my personal opinion).
Anyway, I strongly recomend that thread from macrumors. Right now my iPhoto Library lives in a new 200GB partition and I don't need to fumble around with external hard drives or disk images. But setting up the environment requires some technical background and running some Terminal commands. This is not "it just works".