Telerik will drop in on this conversation on Monday morning, but just to give you a head-start...
The Telerik ASP.NET AJAX controls are all fully compatible with all standard ASP.NET Controls as well as ASP.NET AJAX Extension controls, so there should be no need to worry. If you do experience issues, however, you can rest assured that Telerik and its great community will be available to help out.
The asp:ScriptManager and the telerik:RadScriptManager are essentially the same thing and you only need to have one or the other on the page to "enable" AJAX on the page. There are benefits to using the RadScriptManager over the standard ScriptManager though. Those details are here:
Other resources that may be of interest can be found here:
Hope that helps,