You can drag virtually any video file format onto the device in Rotato
Your video's size should match the device selected in Rotato. You can see the size for all devices in the scene by clicking the 'i' icon next to the scene name in the right hand sidebar
When you drag in a video you'll see a visual timeline of the video under they keyframes. You can use the timeline to time your animations to what's going on in the video.
The exported movie's duration will match the dragged in video, unless your keyframes are longer.