Thank you for writing.
Our RadRibbonBar not supports separator elements. The control is designed to split the buttons in groups and it is not using separators to do this. However you can achieve similar look with customization of the borders of the group, just like we did in the Office2010Back theme for example. You can take a look at our Theme Viewer
application that demonstrates all themes.
In order to create small buttons you should use button group. You can read more about them in this documentation article: http://www.telerik.com/help/winforms/ribbonbar-adding-and-removing-groups-and-buttons.html
Also you can take a look at the attached demo project where I made some customizations of the ControlDefault theme so it will look similar to your screenshot.
I hope this helps. Should you have any other questions, I will be glad to assist you.
the Telerik team