Since the whole idea of mounting with sphag is to increase humidity around the plant and keep moisture on the root system, do you still need to use sphag when mounting in a terrarium? I'm assuming the answer is yes, but probably to use less sphag than you would with a normal mount. Just thought I'd check to be sure.
Depends..... A lot mini's grow with there roots in or on moss in the wild... Some grow with there root exposed. You'll have to research on how they grow in the wild.
I was given a mini-phal recently that was in sphag, so I mounted it to the EcoWeb back wall of my terrarium using spike clips. It is growing its roots out of the moss and into the EcoWeb, so I imagine the moss will drop when I "unclip" it.
I suspect it's a good idea to have the moss present at first, but once the plant gets itself established, it's probably superfluous.
Since setting up my terrarium I've been removing sphag from almost all of my mounts. But whether you want sphag or not depends more on how high you keep the humidity and temperatures, how often you water, and the types of plants you keep. I keep my humidity high and my watering frequent so even small amounts of sphag tend to stay too wet for my plants.