I have three optical drives daisy-chained by firewire, toggling between them is fun.
The PCI card-sleep issue is confined to USB only, no worries with firewire, since it's native to the motherboard in the G4.
Re your combo enclosure, for some mysterious reason, firewire occasionally goes awol. USB is a good back up in that event. A pointer I learned - don't have both USB and firewire cables connected at the same time, it's one or the other.
If F'wire does happen to go missing in action, unplug the power to your enclosure completely and find something else to do. Return to the enclosure after an hour or two, plug it in, and try the firewire again. Power disconnection often restores f/w functionality.