For goofy, something from the exam would work nicely. Oh! How about when they get reunited with Killua at his house? You have some nice Killua/Gon interaction, and he has a good time teasing Leo and Kurapica.
You're favorite is Killua, right? Though that's after the exam . . . Kougaiji would definately have to be from GM Saiyuki (I hate his character design from Reload and Gunlock). Something in which he's summoning. Sorry, I don't know Saiyuki well enough to pick out an exact episode or scene. Maybe go with his first summoning? The episode in which we meet Lirin and Hakkai has trouble with the demon from his past?