What type? As far as American types the Western can be easily swapped to mice in most cases. Easterns and Southerns can be too but its trickier.

I wouldn't risk the fire-belly. I don't know what type of toxin they have but I think its different from a bufo-toxin which is what Hognose Snakes (at least US species) are immune to. The toxin/poison on Fire-Belly Toads is on their skin and does not have to be excreted as far as I'm aware though I may be wrong, which is why I believe it different from Bufo-Toxin.

As far as frogs do you mean true frogs like bronze and leopards? My Eastern will readily eat Bronze/Green Frogs that I catch in my yard during the winter when its harder to find Southern Toads at night.