Fibre does not help everybody with bowel regularity or digestive symptoms. People with digestive conditions, such as IBS, may find that symptoms worsen when they take a fibre supplement. Some types of fibre can irritate the gut lining.
In a recent research paper (American Journal of Clinical Nutrition), it showed that psyllium and pectin (found in apple), respectively, were the most effective as supplements. As for dosage, more than 10 grams of additional fibre or more per day proved to be the optimal supplementation for chronic constipation.
Partially hydrolysed guar gum (PHGG) relieves symptoms in constipation-predominant irritable bowel syndrome (IBS) and may have prebiotic properties. Galacto-oligosaccharides (GOS) has been shown in studies to increase naturally gut flora populations of Bifidobacteria and Lactobacilli. However, they are not always tolerated by ALL.
I advise to build up gradually if trying any form for supplementation, then gradually increase the dosage to avoid side effects.