Drones are a great way to get dramatic footage that you can't get any other way. They can be a great addition to your SlingStudio setup.
But there are some important things to bear in mind:
For drones that have an HDMI-Out port on the drone's controller: You can use that HDMI-Out port on the controller, along with an HDMI cable, to connect the drone controller to the SlingStudio unit's HDMI-IN port. Alternatively, you can connect the drone controller's HDMI-Out port to a SlingStudio CameraLink device; the CameraLink then connects wirelessly to the SlingStudio so that the drone's output can be used as a video source with SlingStudio.
For drones that do not have an HDMI-Out port on the Controller: If you are using a smartphone to view the video feed, connect the smartphone to the SlingStudio unit via the HDMI-Out from the smartphone. (You can do that with an appropriate HDMI cable, plus any needed converter plug, to feed an HDMI signal from the smartphone either to the SlingStudio unit's HDMI-In port, or to a SlingStudio CameraLink device.)
Important: Be aware that, with that last method, what you see is what you get, as the drone's controls appear in the video feed.