I'm working with a tech-savvy cultural institution to customize and deploy an open-source application currently hosted on GitHub. I'd like to use GitHub's reputedly-amazing branching powers to manage their customizations while still picking up upstream bug fixes and enhancements. However, the institution is only familiar with SVN for version control, so I want to educate myself a bit to make sure I'm setting them on a sustainable path:
What are the best GitHub practices for institutions with multiple contributors? I see that both ScholarsLab and CHNM have accounts under those names but also know that a number of people doing development in each organization have personal accounts. Do the organizations have organization accounts with the personal accounts set up underneath them?
How much does an institution generally pay to host 5-20 public/OSS repositories?
How common is repository forking in the use case I've described? In this scenario, the institution-specific work would be configuration files, stylesheets, and possibly some layout editing to bring the app's look-and-feel in line with other web properties. Is there a better--or inferior but common-enough to address--way to handle this kind of customization of an upstream application?
I'm also interested in any examples of large-to-medium-sized cultural institutions or DH organizations who have embraced GitHub.