I know next to nothing about Coco Coir as a grow medium but I would think that if you used it at the bottom of a wicking bed it would quickly rot and compact down meaning you would need to add soil to the top of the bed to make up for the lost space. I don't think it would have much nutritional value for the plants so you would be better off mixing it in to your soil/manure mix to add extra organic matter.
As far as wicking beds go I think most people on here have come to the conclusion that you don't need anything special at the bottom of the bed to wick the water up and that just plain old soil from top to bottom with a membrane like shadecloth stopping dirt getting into your distribution pipes is the best way to go.
If you use straw in the bottom (like I did) it just rots down and dissappears and the large gaps inbetween gravel means it doesn't actually wick water very well, if you want to use a cheap fill to make up the bulk of space at the bottom of a wicking bed I would recommend river sand.
Hope this helps.
P.S. fo some reason your last two topic post have been under the "Pests and Diseases" thread
