Ya, i have the same mice, IMHO they are the best.
So you've run xf86config and you have a XF86Config file but X won't start because the mouse doesnt work, right?
You will need to edit the XF86Config file and change mouse protocol to imps/2 like this:
Option "Protocol" "imps/2"
replacing whatever was in that place. Now try to start X again, if it doesnt work then you need to recompile your kernel and make sure to add USB support and full HID support. If this scares you, then you can just copy over a USB kernel from the slackware install CD and boot that one. Once you've managed to boot the USB kernel with full HID support both GPM should work and with the aforementioned hack to the XF86Config file X should start without problems.
Good luck, although it might look hard, it's a 10 minute job, really it's not a big deal, if you need more help just ask!
Hope that helps