As savoirfaire stated - the Apple Pro Speakers came in two forms : the first kind shipped with the G4 Cube and were USB audio based. Thus, the "soundcard" and amplifier was technically built into the speakers themselves, and the speakers received power through the USB port. However, the G4 cube was specially made to provide higher watts through its USB ports to power the speakers. These would technically work, but since most "normal" usb ports don't provide that much power, the volume wouldn't be very loud.
The 2nd release of the Apple Pro Speakers were some special 2.5mm (i think?) plug which required amplification from the system itself. There were made to work in the iMac G4 mostly (and I think some Powermac G4's - Digital Audio version?). Again, totally different plug and will not fit in a standard 3.5mm jack.
Just about any multimedia speakers you buy today will work fine on your mini's 3.5mm headphone jack.