50 mm Yongnuo lens filter size is 58mm, you can reconfirm your lens mark as "ø" (diameter) symbol, if the number is 58mm, it would be ok ,thanks. 

Yes, it will fit any Sony e mount body, either full frame (a7..., a9...), or aps-c (like a6500, etc) 

yes, the flash can work on Canon 7D 

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. 

yes, i had no problems with infinity focus with this adapter. the only FD mount lens i use anymore with my sony is a vivitar 20mm, and i pretty much only shoot infinity focus with it.