Wyverns don't breathe fire. Dragons do. Wyverns have poison fangs and a stinger at the end of their tail, and only one set of legs. Their skin is leathery instead of scales, which is what Dragons have. Get your facts straight.
Although it is never seen in the game, support conversation between Cherche and Nowi reveals that the wyvern mounts used in Fire Emblem: Awakening are able to breathe fire.
From de wikia.
Wyverns in fire emblem aren't Wyverns from dragon age, they're devolved dragons. Get your facts straight. :p
No where in fire emblem is there mention of wyverns using venom. At least not on the wikia.
Whether or not they can breathe fire in the other series, they can in Awakening, as Minerva burns Cherche with a breathe of fire during their first meeting. Since we're based off Awakening, it's safe to assume wyverns in this rp can as well. :p