Personally, I don't think eating less will do much because after about the fifth time you go to the bathroom, you will be pooping nothing but liquid anyways. If you were constipated, then I would say yes, avoid putting additional food in there because putting laxatives on top of a blockage can give you some terrible pains before you get the hard poop out (hurts the old bum hole too, I can tell you that). But for people who aren't constipated, eating less won't help, I don't think, because you will very quickly run through your food and start pooping water. And poop water, and poop water. Most of what you will poop will be water and tiny bits of food, not solid stools. Why fast extra when it won't really save your bum hole any worries?
I was told that I would only be cleaned out when the water came out clear. Mine never did, but my colonoscopy went over well anyways. My water was yellow because I had a bile problem, not because I still had poop in there. There was nothing I could do to stop the bile!
I think the best advice I've seen for colonscopies are baby wipes in lieu of toliet paper. It doesn't take long AT ALL to get raw down there!
Also, Cottonelle now has toliet paper with Aloe in it. That's about all I use at home. I don't think I've had and major stomach problem since I switched us, but the toliet paper at work is so cheap that even a mild problem can leave my butt sore from it, but it doesn't feel sore when I come home and use my aloe paper. So I think it helps for regular problems; I would still go with good-quality baby wipes for the conoloscopy prep, just in case.