I'm not so sure about the fiber thing. I rarely get 15 grams these days and I've never been so happy with the regularity and consistency of my BM's. I eat Warrior Diet style, so I just don't tend to get in many vegetables, just lots of protein and fat. Back when I was eating six daily meals with bags of broccoli with every one I mostly felt bloated and constipated.
Caffeine is a laxative, that's well established.
Watch the fish oil thing. Umm, a "friend" of mine after megadosing for a few weeks actually started leaking fish oil out the other end. Not a fun experience, or so she tells me
If you want to flush out your colon, google "vitamin C flush"... But don't do it too often.