I've always used just long fiber sphagnum moss.

Sarracenias and Droseras should not be feed any kind of fertilizers. They naturally grow in very low nutrient bogs and adding fertilizers to the medium can kill them. I couldn't imagine coffee being any good for them either. You should be watering with distilled, rain or R.O. water.

If you grow them indoors you can give them the occasional dead bug. you can buy them freeze-dried at many pet shops, but it is best to place them outside during the summer if you can so they can get their nutrients the natural way.