You do realize that (IRL) a sword doesn't actually have to be used in two hand until its about five feet long (probably longer for a Toa). They being said I've heard Kahi put forwards the idea of Kopaka being able to extend his reach with icicles, so a short-sword might emphasis that.
What you are describing is not a long sword, but a full-sized zweihander.
That being said, I think your logic for Tahu makes sense, although I'd give him a long one-handed sword, because I'd imagine he's impatient to meet his opponent, while Kopaka is more willing to sacrifice range for defense.
Personally, I'm in favor of giving Kopaka something like a rapier (with a simplified hilt to fit the theme) and round shield. I feel it fits Kopaka's reserved, precise style much more aptly.