1) 8.6 does have Firewire drivers, and it should be able to drive the card. The manufacturer should have the card's requirements stated in the specs or some place online. While 8.6 will likely run the card, you'll certainly get better Firewire support with 9.
2) Your pre-G3 machine is compatible with 9.1. Apple officially phased out support with 9.2, but functionally there are so few differences it's not a big deal.