I would ordinarily suggest you not put any compost in your mix, but since you're going organic I would go for a 3 part coir + 1 part organic compost mix + 1 part perlite (vermiculite would be marginally better, but since you have perlite it's not a big deal) for a 60/20/20 mix overall (by volume, not weight). Watch your watering, both over and under watering...and mix the container mix very well before putting it into containers. Soil + soiless container mixes are a bit more tricky to monitor than a total soiless container mix. This isn't a "slam dunk" mix...it's just what I would do if I had your materials and I was planning on growing them with organic methods. Ordinarily I wouldn't put compost anywhere near a container mix, much less 20%, but without chemical fertilizers the nutrient leeching vs availability can get a little harsh and the organic fraction will help hold a bit of it around in the root zone a little longer. I would also err on the side of "too little" compost in that mix over "too much" if you're measuring out the parts by sight.

I'm not sure if this is the same stuff, but we use a compressed block of coco coir for bedding in our snake tank. (Yes, we have 3 Boa constrictors as pets.) When we change it I just put the short end of the block in a shallow amount of water and when it rehydrates I break it off and kind of fluff it up with more water. If it isn't enough, I wet and repeat. Nice benefit is that when it is time to change the bedding, the used bedding goes in the compost. I love stuff that has extended value.

What kind of soil layer is this? Any animals in your terrarium? If there are no animals in your terrarium, I don't see a reason to put coir over the other soil, unless you ran out of the other soil. In my experience, moss grows fine on regular potting soil, though I haven't grown any moss on coir to compare the two potting mediums. -Jmcat

I wouldn't crush cans to use as ballast because water can get into the can and support all sorts of nasty bacteria and fungi that don't mix well with plants. Better would be empty soda bottles with the caps screwed on tight, or anything else that takes up space w/o holding water. Bricks make good ballast. If weight makes bricks a poor choice, think of old Tupperware turned upside down, or kids' plastic building blocks. Even overturned plastic pots are excellent at just taking up space. See figure D below. How much perlite is too much depends on what you're growing (some tendency toward fluoride toxicity in many cases - rinsing thoroughly and screening out the dust [use a mask] helps limit the amount of fluoride in perlite), what other ingredients make up the soil, how coarse it (the perlite) is, and whether or not you screened it. Al