Thanks for your feedback!
We agree that the documentation is not complete and need to be improvement. We work on it and the help topics will be updated.
Also about the multiselection feature of RadComboBox, this feature appeared to be more difficult to be implemented than we thought so we defered it.
About the custom skin problem - please, open a support ticket and send us a simple running project illustrating the problem which we ca test locally (including the custom skin).
the Telerik team