Just a note, Horde Battle capturing works like Double Battle capturing - you can't throw Pokeballs when there are more than one Pokemon on the other side. This means even if you run into a horde of 5 Shiny Pokemon, you can only capture one of them (sadface).I think I'd be stuck just catching all four of them and releasing the ones that weren't shiny..
And for everyone trying to determine which size Pumpkaboo/Gourgeist they have, I recommend spending a few minutes capturing a few. I went out and captured about 12 of them and I got at least one of each size. I arranged them from smallest to largest in my PC box and when I'm not sure about the size I just stick it under one of the columns and repeatedly press up and down to compare sizes, switching columns until they match.