I can imagine with the right build of a Squid, it might be possible to get away with a single engi for firefighting (Wrench/Extinguisher/Chemspray lol). Maybe - MAYBE - for a Goldfish too. Any other ship, and you want at least 2 Engineers to have firefighting tools.
Obviously, if there's a gunner on your ship, this means no engineers should be running without firefighting tools. On a ship with 3 (or 4) Engineers, having one (maybe 2) crew members with no firefighting ability is a little more viable. Even then, the flexibility of having both repair and fighting tools is hard to pass up when you have an Engineer. Even 3 crew members can't be everywhere at once, and if you happen to be the guy next to a burning part of the ship, you might end up wishing you were one of the firefighters...