I am not a doctor but my own opinion is this: If I were cleansing using only a fiber product and a probiotic I would be extremely particular about the food I eat.
I would want to ensure that I eat a diet rich in complete vitamin B complex such as oats, barley, wheat bran, salmon, avacado, Brazil nuts etc.. to ensure that I am getting sufficent minerals and B vitamins. I would reduce or eliminate refined sugar from my diet and severly restrict the amount of caffine I am ingesting.
Depending on the amount and type of fiber you are taking you may also need to pay close attention to your electrolytes levels. A good source of electrolytes is a product called Recharge made by Knudsen. Do not use Gatorade as it contains refined sugar.