Are there some guidelines, or does anyone have suggestions on how I should package something that I need to be able to have multiple versions of installed at the same time?
I'm not sure if this fits with what you're trying to accomplish, but on a project I worked on, we did this by first dictating a directory structure, something like:
project_name was the "base name" of the project, and
version was a specific tagged release. We then used an arbitrary number for the RPM version (monotonically increasing), so that releases within the same tag or branch could be RPM-upgraded without conflicting with other tags/branches. This means you need to encode a version number somewhere in the RPM name header, rather than the version, e.g.
Name: foobar-1.0 Version: 1234 Release: 1 # ...etc...
In this case the RPM might install to under the path
/usr/local/foobar/1.0 and subsequent versions would be named "foobar-2.0" and be installed similarly.
The name "foobar-1.0" implies that you need some other system, not the RPM system, to keep track of version numbers - we used a shell script that simply generated the specfile on demand for a tagged release. This can be a bit cumbersome, but it buys you the flexibility of being able to install multiple versions of a package in a common way, and be able to use more than one at the same time.
This is just a suggestion, and YMMV obviously.