I don't know about fire breath, but Rodan was a bad azz!! Totally capable of more destruction than Godzilla, and thus, a bigger threat. Why? Because he GETS around a lot more. Godzilla moves at like 30 mph on land and his atomic blast, though radioactive, is relegated to a small area that may or may not spread and cause crazy damage. Rodan can fly all over the place and if his flying in low altitude your city is razed blocks at a time, son.
Blocks.
At a time.
Bye bye mother and pops and old pa and sparky!! Bwaa haa haaaa