If you've maxed out the ports on your mini, then you might not have enough power available through the the USB ports to power more devices in the chain. A USB2 hub with it's own power supply would provide additional power for the extra USB devices, and reduce the power load on the mini.
Real Macsters don't repair permissions.
Opinions are like toothbrushes. Everybody has one, so there's no need to share. - Red Green
Keyboards and mice are all 1.1, because there is absolutely no way possible for either to use up the bandwidth of 1.1, let alone 2.0. You could have ten crack-crazed monkeys banging on the keys of ten keyboards as fast as possible simultaneously, and it still wouldn't get to USB 1.1 speed.
It's similar to "the internet" (which cannot possibly use up the bandwidth of a 10-base-T ethernet) or an optical drive (which cannot possibly use up the bandwidth of even a lowly 33 MHz version of an old IDE bus).
You should have your KB + mouse plugged directly into the Mac. Do not use a hub with either.
Use a hub on (one of) the other USB ports on the Mac, and plug your 2.0 stuff there.