The only QR code that will actually give you a Pokemon is the Magearna one. The others merely register a Pokemon to your Pokedex as if you've seen it (but not captured it), and count towards the daily Island Scan total. Thus, you can't receive any Pokemon you want from QR codes. However, if you scan a shiny Pokemon's QR code it will be permanently entered in your Pokedex, so you can have a Pokedex full of shiny Pokemon that you never actually obtained.
