- Yes, you basically need to add a Use statement for each class or use FQNs.
- Yes, starting your classes with some unique domain is generally a good idea.
- Take a look at php composer. It's designed around the notion of being able to use libraries from multiple sources. It includes support for auto loading. Better than copying.
Consider taking a look at some projects that use namespaces. Doctrine 2, Symfony 2, Silex etc.
Namespaces can make your code easier to read and maintain by listing classes used in given file along the top (use statements) and then allowing for shorter class names (aliases). But stick to using them the way the php folks intended them to be used. Lots of people get into trouble trying to apply namespace concepts from other languages.