I'm in the process of trying different linux systems and am currently trying out Slackware. I wanted to set up one of my secondary 120GB hard drives for Gentoo, openSUSE, Fedora, Mandriva, Kubuntu, and Debian. I want to decide on what's most comfortable for me. I remember having a problem setting up that many partitions in the past. Any tips/free apps anyone can mention? Also, can anyone think of solutions for problems I may run into installing these on a secondary hard drive? I'm using LILO on my primary.

I realize this is somewhat broad, but any help would be appreciated. I'd like to run into this feet first.

Oh, and one other thing, LILO's kernel line for Debian refers to the mount point from within Slackware. Should I change that? It does work.

This is easy to set up---I have a system with maybe 10 Linuxes (I've lost count..

First, why 20GB each? Unless you install every application in the book, you would have a hard time filling even 10GB with a Linux install. For evaluation and experimenting, you have the parttion sizes ~ 6-7GB each. Leave the rest unformatted--or maybe set up a shared data area.

Since you are putting only Linux on the drive and you will have more that 4 partitions, I would suggest this structure:
#1 primary ~7GB
#2 extended fill all remaining space
#3 swap ~1GB at the end of the extended part. (ie at the end of the disk)
install your first distro on #1
When you install other distros, let their installers create more logical partitions as you go.

(NB--one reason I suggest this is that I once found myself with an extended partition that did not go the end fo the disk. The partitioning tool of the moment would not add more partitions, but also refused to resize the extended partition. I THINK it was QParted that bailed me out..)

Bootloader:
I prefer GRUB---If you stay with LILO, then don't allow any of the installs to install a loader. You will then need to edit the LILO config file to point to the new installs.

Why put the swap at the end? Everything I've read claims it runs better at the front.

Oh, and I put 20GB just for leeway. I have a very bad habit of filling space. That's just an estimate though. Thanks for the space suggestions.

Forget swap placement - if it's sharing the same disk as your OS, the issue is moot.
As for the loader, LILO will drive you nuts. Go with grub.
Me, I use 10 Gig - nice round number, easy to remember, and disk is cheap. I pre-allocate a few, and just re-install something else over the top if I decide to "toss" a particular distro.
Be particularly carefull if you decide to add Solaris to the mix.

To get it out of the way???
I really don't know---this what I have seen many people do.

How would location on the disk make any difference? Purists will not like this, but--at some level-- a harddrive behaves as a random-access device. (I just did an experiment reading from an 80GB drive using dd. I could not see ANY difference in execution time between beginning and end of the drive.)