Depends on the impeller I think, in my case the blade worked fine. If it's stiffer/stronger plastic then maybe a dremel.
The pictured venturi is intended as a replacement of the "stock" venturi .. (yes it would go onto the pump intake
) I'm sure it's an improvement whether the skimmer is recirc or not .. the idea here is just to get more air into the skimmer.
I could be mistaken, but I get the impression that it's only the larger skimmer folks who are using an airpump to drive the air injection.