There's certainly a case for having Austin start at number 9, as he does bring a good running game. Luani's not technically a bad hooker though, and he works hard in defence.
No way would you run Koirobete at fullback. There are some wingers who do well moving to fullback to get more involved in the play. Koro would not be one of them. He misses too many tackles even on the wing and has had some very inconsistent performances under the high ball. I seem to remember a game last year where he dropped four or five bombs, and his overall ball handling isn't up to first grade standard.
I really want Koro to work out as a first grader. He's nigh unstoppable once he pins his ears back with the ball, but just about every other aspect of his game is wanting.