Tetra is a great make, although I probably wouldn't use a product designed tor fish on amphibians or reptiles without comparison to the ingredients in Zoo Med Reptisafe which is what I use. It is probably fine which I am sure others will confirm or deny, but I would be cautious without such a comparison or confirmation.
Your substrate should be damp not soaking wet, mud like or bog like. If your substrate becomes like a bog it will stink and produce nasty bacteria which will be detrementa to the health of your frog. Idealy your substrate should have a drainage layer underneath which is kept seperate by a fine mesh or the type of cloth used in gardening as a weed barrier.
The "pill bugs" you have heard about are not exactly the same thing as is used (although they can be in certain circumstances). The usual custodian insects to add are tropical woodlice which although they are much smaller, they are still from the same family of insects called isopods. Tropical woodlice are a better option as they are better suited to the higher temeratures an humidity than "pill bugs" are. Custodian insects clean up the substrate by eating any mould and the fecal matter in the substrate. Although in your case as your substrate will require regular changes, it probably isn't worth adding them.