OK, this isn't exactly the same - but I recently upgraded my Powertower to a G3, and have installed both a Firewire (Orange-MicroLink) and a USB Card (some random brand off the shelf) via my PCI slots. The USB Card worked fine straight out of the box just using Apple's USB drivers with OS 9.04, and the Firewire came with some sort of wacky Orange proprietary software/extension that works as well. No conflicts or problems (except my HD isn't fast enough to pull digital video from my dv camera). If i was a betting man, i'd put my money on the Firewire portion of your card as to what's causing problems.
Sorry I haven't a solution, but you wanted to know if anyone else had tried what you had...and it's possible!