In order to use your tools, will I have to reboot in OS9 or something?

I've been using Martin's tools in Classic since I first started running Mac OS X. (Along with Poser 4, which worked fine despite technically not being "supported" by CL.) If you're sticking with Poser 4 you'll need to keep your Mac OS 9 System Folder anyway to start Classic. With Poser 5, MacConverter isn't needed as much. However, you still need to convert rsr and pef files. (Though pef files are relatively uncommon, and rsr files are slowing being replaced or at least supplemented by PP/5 png previews.)