I tried those pills and they made my constipation worse and my diarrhea worse, but some people take them without a problem. Like everything else for the guts, you just have to try them and see if they work for you.
Fiber does necessarily = loose stools. Fiber is supposed to regulate you, regardless of what type you are. Insoluable fiber adds bulk--something you need for loose stools--while soluable fiber helps lubricate the intestines (or the poop one), to make it easier to pass stools--something constipated people need. I have read that it's best for people with diarrhea to take less soluable fiber and more insoluable fiber; extra soluable fiber can make you worse. That's why sometimes you have to try out different brands to find one that helps your particular case.