I think Torchwood is a tricky one in general because Jack himself is wildly OC and different depending on who he's interacting with and where in his timeline you are placing him. You place him with his team and he's easily the leader, but you put him with the Doctor and he goes subservient. I don't think you will see two fanfic writers portray Jack in the same way. My flirty and eccentric and somewhat needy Jack is different from your bitchy and dark and selfish Jack (who I LOVE) and I think both those Jacks are fairly canon. You know?