It depends on many factors, the mode of your lens autofocus/whether you use flash/whether you shut down after taking the picture or wait for the camera to automatically sleep 

No, you can't sync two of these to use. 

yes, the flash can work on Canon 7D 

This will work on any 82mm lens. 

There are 6PCS adapter ring included in this flash, here is the size:
52 55 58 62 67 77mm, as long as your lens size is one of them, then could be used,
Here is the way to check your lens size:
Your camera's lens thread size will be marked somewhere on the lens barrel or printed underneath your lens cap. This number is always preceded by a "?" (diameter) symbol. For example: ?52 = 52mm lens thread size.
If your lens size is none of them, please tell us, we may include the size you need but at this case, this item must be shipped from China so that we could pack with this adapter ring you need.