If you right-click on the component in the topdesign and select the "Datasheet" choice, it will open the PDF datasheet for the CapSense. This should contain the information you are looking for. Here is a datasheet for it I found online, but it has version 3.10 which may be outdated: http://www.cypress.com/file/127731/download
From a quick skim of the document, it looked like the minimum scanning frequency clock could be as low as 3 MHz, but that is lower than what you are able to set it, so I suspect that information I found.
Yes, you are right. The information regarding modulator clock frequency is available in the CapSense component datasheet.
For third generation CapSense devices, the minimum modulator clock frequency is 3 MHz.
Whereas, for fourth generation CapSense devices (4000S, 4100S, 4100S Plus, Analog Coprocessor), the minimum modulator clock frequency is 6 MHz.
Thanks and regards,