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..)
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.