I really like mini!Jack stories - when done well I think they give an unique insight into the full grown Jack.

I found when I was writing this that to keep Jack in character, I write adult Jack -- he's just shorter. Because that's how mini-Jack feels. (Short and annoyed :P)

I can just see Jack not liking Edward at all, as he's everything Jack isn't anymore

I would argue that Edward is everything Jack loathes. Edward kills for money, and does everything for himself, not the greater good. Jack *could* be Edward, but all he ever did was for country and command. Not all of it nice, but different reasons.