2) Probably. So long as you're not gonna have an install frenzy, then 3G is OK, I guess. I'm running with 6 at the moment, but I like a little freedom to play with software.
3) If you have a partition already set-up under SuSE, then make a note of what the partition number it is, and you should be OK.
A word of caution, though. Do not install the Slack boot-loader. Stick with your SuSE boot-loader for now. There's a couple of threads around on how to triple boot. In summation, you'd boot into SuSE, mount your Slack partition, copy the kernel into your SuSE partition, edit your lilo.conf/grub.conf file appropriately, save run and reboot.