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

Regular potting soil. Reading through the forums, it seemed like most people were recommending a layer or coir or a lot of peat mixed into the soil for moss.

There won't be any animals in the terrarium.

Thank you!!

There probably is no rule as to depth. How about experimenting. Have some 1 inch and some less and plant with roughly the same amount of moss and see which does best!
Coir is the media of choice for growing mosses. My own experiments seem to indicate that 1/2 to 3/4 inch deep coir has better growth than much thinner depths.