Addressing your first question about the base class, the best solution for you considering that you want all the classes to inherit from one and the same best class is to modify the code generation templates to automatically include the inheritance. We have a code library sample demonstrating this approach.
As an alternative, you can create a base class in the Visual Designer, but this is mostly applicable for scenarios where just a few classes will inherit from it.
To answer your question about Fluent API - it does offer more flexibility in your implementation and if you feel the Visual Designer approach with an auto-generated domain context is too constraining for your scenario, this is the API that can help you build your application exactly the way you want. The disadvantage is that you will have to code your entire mapping manually in a declarative manner.
For more information, check out the documentation articles and videos for the Code-Only approach.
Do not hesitate to contact us if you need any clarifications.
All the best,
the Telerik team
SP1 for Q3’11 of Telerik OpenAccess ORM is available for download.