Soluable fiber is fiber that dissolves in water and makes like a lubricating gel. It is very good for people with C. Insoluable fiber is the tough, stringy bits that won't dissolve at all, and they add bulk, which is good for people with D. All fiber has a some of both types, but you can typically get a fiber supplement that has more of one or the other; you should look for one with more soluable fiber than insoluable fiber. Also, don't do the full dose at once; too much fiber all at once will bloat anyone. Start off a 1/4-1/2 the recommended dose and gradually work up.
Have you tried things for the bloating, like Gas-X? Also, have you tried probiotics? Those are good bacteria. Some people find they work wonders on their bloating, and even on their C. Start out light. If you can do dairy, try the Activia yogurts. If you see some help, work your way up to Digestive Advantage or Culturelle (there are a lot of probiotics on the market, but those are the easiest to find because they're in most any drug store) and see if they help you more. Like fiber, they can cause some inital bloat (the yogurt is so mild I don't think anyone complains about it bloating), so start with the smallest possible dose and work up. Starting out on the yogurt will help a lot to transition to the pills. Also, if you drink milk, switch to the acidophillus brand.