RAD stands for rapid application development. With this methodology you use structured techniques, and prototyping to set your requirements, get prototypes quickly and use minimal planning. It's essentially one process that wraps several stages into one (requirements, design and possibly even the development processes.) It's close to the agile mathod but the main difference is the use of prototyping.

What do you do if one site requires a specific version of a module, while one of the other sites requires a different version of the same module?

Because of the way Drupal is constructed, you cannot install two versions of the same module in the sites/all/modules/ directory. Attempting to do this will lead to fatal errors or, even worse, subtle bugs that arise from certain loading errors.

Drupal is a free and open source CMS written in PHP and distributed under the GNU General Public License. Drupal stems from a project by a Dutch university student, Dries Buytaert. The goal of the project was to provide a mechanism for Buytaert and his friends to share news and events.

Buytaert turned Drupal into an open source project in 2001, and the community readily embraced the concept and has expanded on its humble beginnings, creating what is now one of the most powerful and featurerich CMS platforms on the web.

A traditional waterfall method breaks the workflow down into stages. You complete one phase of the production life-cycle before continuing to the next. The names of the steps are up to you and can vary and are usually tailored to your organization's style or terminology already in use. Make it work for you. The objectives of the waterfall method are to complete one phase before moving on to the next and to have the output from each task feeding into the next so that there's a logical progression of development.