Regarding USB2 support, check the drivers available from 3rd party USB card options. If one lists OS9 compatibility ...
About keeping OS9 available. Yes, it would be a very good idea. Should you ever want to add a CPU upgrade card, you will need to update your Sawtooth's firmware. The firmware update will only work from OS9.
OS9 will also let you keep your old software collection. Most older games were not updated for OSX, and games in general are famous for not working properly in Classic. BTW - for those programs that do work in Classic, you still need OS9 installed for Classic to function.