Mhm... I'm not sure about that, myself. Traditionally, the avatar [i]must[/i] follow the cycle of Earth, Fire, Water, Air. I know that Korra didn't, since she had the most difficulty learning air, but Aang trying to learn Fire out of cycle had a few consequences...

EDIT: Whoops, no, that’s wrong! Korra learned in order as well, given that Air precedes Water in the cycle. Basically, Rosai ought to learn in the order of Earth, Fire, Air, Water.