We assume that you already have:
One "main" or "primary" Linux distribution installed that is running a 2.4-series or newer kernel,
a free partition with 2--4Gigs available, and
sufficient disk space for the secondary installs (and any subsequent development/testing work).
Given those assumptions, the following outline is one way to setup the multiple secondary Linux distributions that will be used for development, testing, or other purposes:
First, create or locate an unused partition on one of your hard drives that is large enough (usually 2--4Gb is sufficient) to do a basic install of one of the secondary Linux distributions.
Install the secondary distro into this partition but do not add it to your boot configuration. Note that this install need only include the packages that your target application (or tests) require. For instance, you may be able to ignore applications such as the X server or other space-hogs.
Reboot your primary Linux distribution and mount the partition containing the freshly-installed "secondary" distro. Copy (preferably using tar -cp or some other method that preserves permissions) all the files from the secondary distro to a location such as /opt/distros/DISTRO_NAME.
Repeat steps 2--3 for any additional distributions that you wish to install. The result should be a directory structure resembling:
/opt/distros/redhat_6.2/ suse_7.2/ mandrake_8.1/ debian-potato/ slackware_8.0/
where each directory contains the complete set of files resulting from each distribution install.