Tana is best as a Wyvern Lord... You shouldn't waste the overall power and speed to just a FalcoKnight.
As for Cormag, he's been always a Dragon Knight even experts agree to that.
By the way, to all Fire Emblem n00bies here, the weapons aren't named out of thin air.
Ereshkigal (Tome of Nergal from Blazing Sword) - In Babylonian mythology, Ereshkigal was a goddess, wife of the deity Nergal. Together, they both ruled the underworld and governed the dead. Nergal was also the god of war and pestilence, and was a solar deity who represented certain phases of the sun.
Gespenst ("S"-Level Dark Tome in Blazing Sword) - From what I can decipher in my searches, Gespenst is a Dutch word for "ghost" or something similar. (Do a search for "Gespenst" on Google to see what I mean. There is also an entry for Gespenst in the Dutch version of Wikipedia.com)
Fenrir (Dark Tome) - Many should know who Fenrir is. In Norse mythology, Fenrir was a terribe beast in the form of a massive wolf. The Gods feared Fenrir because prophecy stated that he and his family would be responsible for the world's destruction. As a result, they caught the wolf and chained him to a rock deep within the earth.
Gleipnir ("S"-Level Dark Tome in Sacred Stones) - The name of the unbreakable chains that bind Fenrir to the rock Gioll. The gods ordered the dwarves to forge a magical chain so strong that it could hold Fenrir. The thin, soft ribbon Gleipnir was the result, and despite its appearance, it was incredibly strong.
Naglfar (Tome of Lyon from Sacred Stones) - In Norse Mythology, the terrible ship Naglfar belonged to the sea giant Hymir, forged completely from nails of the dead. When the flood that precedes Ragnarok comes, Hymir will use Naglfar to sail toward the battlefield of Vigrond. (Ragnarok is the time when destruction will break loose in Norse Mythology, and as a result, even the gods will meet their demise. It is mainly used to refer to the massive war that will break out amongst the gods and the giants)
Excalibur ("S"-Level Anima Tome) - Whoever doesn't know what Excalibur is should be shot. Anyway, just in case, I'll say it. Exalibur was the name of the magical sword owned by King Arthur in Arthurian legend (why it would be a magic Tome instead of a weapon in Fire Emblem, however, is completely anyone's guess).
Fimbulvetr (Anima Tome) - The winter of winters that will precede Ragnarok. There will be three Fimbulvetr winters with no summers in-between.
Nidhogg ("S"-Level Bow in Sacred Stones) - In Norse Mythology, Nidhogg is the monstrous serpeant that gnaws at the deepest root of Yggdrasil, threatening to destroy it. It lies in Niflheim (the realm of death) and eats corpses to sustain itself. (For those who don't know, Yggdrasil is the tree that links and shelters the worlds of Norse Mythology)
Garm ("S"-Level Axe in Sacred Stones) - In Norse Mythology, the monstrous hound Garm guards the entrance to Helheim, a part of Niflheim. Garm can be compared to Cerberus from Greek mythology, the guardian of the underworld. When Ragnarok comes, it will join the giants in the battle against the gods.
Vidofnir ("S"-Level Lance in Sacred Stones) - Vidofnir is the golden rooster that perches at the top of Yggdrasil.
Audhulma ("S"-Level Sword in Sacred Stones) - The divine cow that belonged to Ymir, the primordial giant in Norse Mythology.
Reginleif (Ephraim's Professional Lance) - The name of a Valkyrie in Norse Mythology. (The Valkyrie were female warriors who led heroes slain in battle to Valhalla, the Hall of the Slain, presided by the god Odin)
No matter what game you play, one can always find some reference to mythology.
[source from FESS]
Durandal ("S"-Level Sword in Sword of Seals; Eliwood's Legendary Weapon in Blazing Sword) - According to French Legend, was Roland's sword. It previously belonged to the trojan Hector (killed by Achilles). When losing a war, Roland tried to break the sword rather than surrender it to the enemies. He couldn't break it. (Roland is also the name of the founder of Lycia)
Sigmund and Siglind (Legendary Weapons to Ephraim and Eirika in Sacred Stones) - Parents of Sigurd (Sigurd is also the name of the FE4 Lord)