Hi.
However I'm not quite sure how I get the boot menu to present me with the alternative versions.
Install grub or lilo, and come back if you have any problems. Many installers will set this up for you, but I've never used Slackware so I don't know. Hint: it's a good idea to put /boot on its own separate partition or things can get confusing.
Will I have a problem because of the second (new, 10.0) version being beyond cylinder 1024 or whatever the limitation is?
Shouldn't think so, haven't seen any of those kind of problems for a long time. Maybe if you have an ancient bios or something.
Good luck,
Joe