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: