Fire bellied toad lighting and filter.

[FinnyBird]

If the tank maintains a fair temperature, there is no need for a basking spot. I would suggest having a warmer side though if you could manage it, as the toads are ectotherms and would better be able to maintain whatever temperature is appropriate. A twenty gallon could house 4-5 I would say, if you set it up right. It would most definitely fit three without a problem. I would personally suggest making your own filter. They are more efficient and easier (and cheaper) to maintain.
Rocks and sticks should be fine so long as you find a way to make sure there are no bugs fungus or anything else potentially dangerous. You can freeze them or boil them to achieve the same effect. I have never bred my toads, so I'm afraid I can't help you there.

[Duriel]

I've got a number of medium rocks from my area in the tank with my firebellieds. I rinsed off the dirt (and some kind of slime mold) and boiled them for 5 minutes. No reason to waste gas/electricity boiling small-medium rocks. Going to revise their setup next Friday (currently just water with rocks above the water level. Thinking to do like my 40G breeder that my bullfrogs live in and silicone in an elevated land area. Mine spend most of their time in the water, so ~40% more water space while still having land space (and not just blowing money on a larger aquarium) seems ideal.