There is the story of Crohn's boy, which you can find here
I have tried it myself. I can't say how much it helped because I was well on my way to recovery by the time I tried it (primarily as insurance). It certainly didn't hurt, and I wouldn't hesitate trying again if necessary.
Regarding the process, with practice comes good technique. After a while you will get the hang of it. In terms of the amount of water, I would recommend however much you can handle comfortably. If two cups seems too much, start with a cup and go from there. Make yourself as relaxed and comfortable as possible.
I also wouldn't worry about
mixing different powder strains. They very well may be competitive with each other, and one may eventually come to dominate if allowed to interact over a prolonged period of time, but I know of no harm that will come from mixing strains together.