The issue of flax vs. psyllium is a matter of the type of fiber that it is...psyllium is a soluble fiber meaning that it absorbs a lot of water and then gels and expands. Flax is a combination soluble/insoluble fiber...it doesn't absorb nearly as much water and also doesn't expand like the psyllium...hence it doesn't produce the same degree of sweeping action that the psyllium does. Flax is good as a general fiber source, but not nearly as effective for most people as a colon cleanser.
Coordinatng different cleansing along with eating can get tricky because of the timing. As a general rule don't take the shakes within an hour of herbs, medications, supplements, etc. If you're doing more than one type of cleanse at a time it usually requires working out a schedule and following it real closely. In some cases this can be reason enough to do one cleanse at a time.