First of all, I don't ran Linux on an Old World Mac at the moment.
But I have some Old World Macs (PM 7500, PM 9500, ANS ....)
In my PM7500 I had a USB-Card for a while an I had also trouble to get it to work. (under Mac OS) What I remember, you need an OHCI-Card, UHCI-Cards didn't work for me.
I dont't maybe Linux can handle both.
Hope the helps
greetings from Germany Ronny (excuse my sometimes bad english)