Well, I would say that Slackware 9.1 is an excellent linux distribution. However, I am a Unix administrator and, without blowing my own trumpet, am pretty comfortable with editing files, the file structure, mount points and so forth. I mention this because Slack is not the easiest distro for inexperience users. For example partitioning your disk(s) is somewhat more onorus than in, say, mandrake. I would suggest using ext3 with a root partiontion, one for usr, a small swap and then one for /home. Now I know you can do a lot more, but i think that is fine for most people.
It won't be automatically added to your lilo.conf file as this will be stored in /etc in your Mandrake distro. You can add it in however - you need to put in /boot/vmlinuz etc as per mandrake. However, I would let Slack install lilo in the root partion of where it is installed (i.e not the MBR of the first disk). Then you can just added a new entry to you mandrake lilo.conf (similar to the one for XP), which will take you to the Slack lilo. I know this is messy, but I think it is much better to get slack up and running before you start worrying about things like that... (IMHO) In case you don't know after you've edited /etc/lilo.conf you need to cd to /sbin and run 'lilo' to write the file back to disk. BTW, 10Gb is more than enough space for this distro.
Would I recommend Slackware??? Yes and no. It's definately faster that Mandrake for starters. Building kernels is more straight forward, it's more stable and the pkgtool command is pretty cool. I have Slack 9.1 on and 850mhz Amd with 384mb RAM and it runs great. If you have good unix/linux knowledge/skills, you can really do a lot with this distro in terms of configuration and fine tuning. However, it's harder to install than mandrake, there is no control panel like mandrake, as standard it doesnt use supermount (you have to mount CD/DVD's every time you chance disk), you have to use the command line to edit files more.
Basically I would say that it is considered as a 'proper' distro for people who are interested in Linux (although using kde/gnome isn't exactly rocket science). It really depends on what you want from it. If you want to increase you knowledge then definately do install it. if you just want to use linux and not have to think about it, then it probably doesnt offer much above mandrake. Personally, I think it is a great distro that is stable, well supported and very configurable. I would definately commend the Mandrake people for bringing Linux into the mainstrean tho.
Go for it I say! You can always format the disk. A little tip - you could always format the partions before hand using 'disk drake' in mandrake and installing Slack into those partitions after a re-boot. People seem to find that a stumbling block with this distro (not sure why).