They could use a hippo for appa

hehe nah, Appa and momo will be computer generated, and will look awesome.
About the gAangs costumes, I don't think they will be the same as the show. In the show, almost all the costumes are very brightly colored, but in the movie, Aangs BRIGHT yellow and orange would look silly, especially next Kataras BRIGHT blue. In general, they are probably going to use more pale and diluted colors, and for Aang, I think new colors will be used, but still one that give that Autumn feel, maybe pale lavender and fushia or something, I don't know