Well, it seems kind of natural to me. However, as to Éomer's words, "guises" may be just simply a disguise for an old man who goes to pick some wood to Fangorn, as we know it from the book. (Okay, maybe not exactly pick wood...) So not really any shapeshifting, just maybe some "spell-enhanced" change of clothes and such. Gandalf tells Gimli at Isengard that Saruman may choose to look like Gandalf in Gimli's eyes if it fits his purposes, however I believe this too doesn't refer to actual shapeshifting but to mere seeming, the same as his Voice - after all, he sure wouldn't seem the same to everyone's eyes, and it would be physically impossible to achieve that by shapeshifting.