Love to see if there was a way to get a source into two categories.

This way, for example, one could create categories for all types of sources (EG, cameras, graphics, etc) and then also be in one where there are only those needed for a program and others hidden at the time.

It's a standard feature of vMix. It allows you to group inputs any way you like, in the settings for every input is a "Category" item followed by a number of colored squares which is the categories you can select.

Each category can be given a name (right click on any category color which is the row of colored buttons above the list of inputs in the main vMix screen) and any input can be in one category but only one category which limits it's usefulness. Once exception is the Black "All" category which is the default and always has all inputs in it making it the place to find any input you may have forgotten the category of, it is also the default category that vMix always selects when you start it up.