Actually, the SoftDevice is not really an RTOS. It does not provide most of the infrastructure that goes with it, and it was designed to be RTOS agnostic. You should be able to run FreeRTOS on top of the SoftDevice.
I'm not sure I understand what you need to know. FreeRTOS uses SVC calls, and so does the softdevice. The softdevice does however not use all SVC numbers, and as detailed in the SDS, some are available for the application to use, for example by FreeRTOS.
