RE: does it matter what form the fiber comes in?
Very much so!!! Different fibers work differently in different people. If you have dry stool once fiber can make it more constipating and if you have wet stool another can make it more runny. Talking from personal experience.
Best thing to do is experiment. If you find one that works you will see results with in 24-48 hours. You may need more or less depending on your body. Once you get flowing you maybe able to cut back too.
There are a few fiber products listed in my thread.
http://www.medhelp.org/posts/show/660767