Welcome to the forum . I would say firebelly toads are fine so long as they are a look but not touch pet. Washing your hands properly before and after handling them is key.

Yes, a 10 gallon can house 2 firebellies. Any more than 2 and I'd recommend a 20 gallon long.

Any ratio is fine so long as they have at least a large water dish or pond. They love to swim. 70/30 would be fine.

Peek around at the vivarium section here and I bet you get some great ideas .