Why a whelk...

I'm not totally clear on what you or rm mean by "information balance," but I do think one characteristic that makes Ianto very compatible with Jack is his apparent ability to cope with having less than complete information. You mention Jack blurting out stuff to Ianto in Fragments, and what I found notable about that was not just Jack blurting out all that stuff, but Ianto taking it all in with barely a blink. He doesn't hound Jack about the wound that stopped bleeding abnormally fast, 51st century pheromones, or having dinasours for breakfast. Knowing Ianto, I feel he would have tucked those pieces of information away, to be called up later at a moment's notice if he found any other relevant info to tie them to. But in that moment, he seems satisfied with just focusing on the task at hand. In KKBB, while the others are grumbling about Jack never telling them anything, Ianto just points out that it's more fun when Jack's around. I think this was also in KKBB, but there's a scene somewhere where Jack asks Gwen to accept him for who he is now -- and I remember thinking that he'd never have to ask that of Ianto, because Ianto already does. Ianto just strikes me as being comfortable with whatever Jack decides to reveal to him without needing to ask for more, and ironically, that seems to cause Jack to relax and tell Ianto more than he tells anyone else.