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:
In this case the RPM might install to under the path
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.
Just give the packages different names, and make sure they install files into separate locations.
Or bundle all the versions into a single package.