I am not well-versed in SPARC hardware, but I believe Solaris 9 should support your Ultra 10, including the network interface. I have Solaris 9 running on a SparcStation 10 and a SparcStation 20, which I believe are more legacy than your Ultra 10. I also cheated and added a SCSI/HME (100Mb/s) S-Bus card to each, so I rarely, if ever, used the on-board Ethernet.
As for the PCI USB, it should be an easy trip to the HCL on Oracle's site. I only worked with one PCI-based SPARC system in my time, and it used some 3Com-looking card. Sadly, I was not involved in its set up.
Good luck.