Why not? It's perfectly possible, especially since there are fairies and races with a single flying unit – it's not something new (dwarves, demons, dark elves, orcs).
Using the same logic, you could put an Archangel on level 1 or 2. After all, in Disciples 2, a unit-healer (or rather, a hero) of roughly the same strength was called that. The game is just as "similar" as a Roc is to a Phoenix.
P.S. I'm still reading this thread and I don't quite understand why it was necessary to transfer almost all the units from Heroes 7 here. Wouldn't it be easier to just play Heroes 7 if you're more interested in the canonical storyline than gameplay issues?
Added after 8 minutes
It's time to think about how to implement the process of extracting models from Heroes 7.
Wizard Necromancer, have you contributed to this yet?
Блистает там воздушностью прогалин.
Но между кедров, полных тишиной,
Расщелина по склону ниспадала.
О, никогда под бледною луной
Так пышен не был тот уют лесной...