I have never done P&B, but tried Colonix for may be 3 days but hated the feeling of the fiber. I started following Andreas Moritz and he suggested Oxypowder and I always liked it but now don't need it anymore.
Here is a link from Andreas forum about colon health, read the whole thread, a good discussion.
(May be I should add he is not for P&B he finds it too rough for the intestines).
Some people according to Chinese and Ayurveda medicine have "dry" bodies and have trouble with things like P&B.
In my time there was a person who did the PB and I saved the link since I thought may be it would be helpful for other people.
The idea to do colosan/ Oxypowder first sounds like a good idea.