Hi Katy, great post. I enjoyed the reading; It shows that you know what you are talking about.
Although I have some clarifications to make (I hope you don't mind)
A unit test is the smallest test you can do on your code.
That depends. When you work with TDD or TCR you write even smaller tests (micro tests) and all together they make up a unit test.
The perfect unit test class will only test ONE class.
Many people associate that idea with "unit", which must be a class, function, or module. It is actually a behavior in which the behavior could be modeled by the interaction of multiple classes or functions. It doesn't necessarily mean having a to test just one class, module, or function.