Yes, I'm sure everyone is aware that specialists are better at what they specialize than people who do the same things and extra. You can learn how to bend with a weapon or two in your hand. It's not like she has to spend an hour earth bending, and spend an hour weilding a hammer. Do both.

I did make Jangbu a little older than Kasumi, and I'm not going to try to make him out to be a prodigy. Though maybe adding on another five years would make more sense.