The most relevant Toady quote I could find on subject was the devlog from Feb 6, 2009.
Today was material breath weapons. For now, there can be forward puffs of gas/vapor/dust of any material, as well as clouds surrounding and liquid/solid glob projectiles. This can be extended to things like cones and so on later. I tested out these and the poison effects by making the berserk test dwarf breathe dwarven blood vapor that was linked to an inhaled poison effect that caused immediate pain throughout the body. He wasn't immune to it himself and ended up passing out before I did.
1) Depends on the properties of the metal. For vanilla metals, it'll probably be either metal powder or a big ol' glob of whatever, but you could easily mod in a metal that's liquid or gas at room temperature.
2) I don't see why it would, but you could easily attach a contact venom effect to the blood in question. Maybe the vomit too, but I'm not sure.
3) Propably not vermin breath. Water breath, maybe, since you can use ice as a material type in the current raws and ice is liquid at room temperature, but it'd be a glob of water (like from a bucket) rather than real fluid water. Toady mentioned Dwarf Lung men in one of the other devlog updates, so you can probably make Dwarf Skin breath or whatever. Probably Ash too, and Mysterious Substance if that remains in the game. Dunno about the plants, you probably can.