Hydrophyte suggested this rainforest mix soil
. I looked into some ABG mixes too, and wasn't sure about the addition of charcoal. I wasn't sure how good that would be for 'pedes, but apparently it's fine for terrestrial isopods, who pretty much have the same diet and requirements.
I don't imagine soilless substrates would work well for millipdes, considering they live in/eat soil.
I'll go check out the herp section more carefully for bug info, thanks!
What does ABG stand for, anyway?