Harmony - Troop Symmetry

It's impossible to have no flying units at all, and if a phoenix is going to belong to a faction (not be neutral), it should definitely be a Light Elf unit (even though it's associated with fire rather than earth).

Why not? It's perfectly possible, especially since there are fairies and races with only one flying unit (dwarves, demons, dark elves, orcs).

In Heroes 6, after defeating the Thunderbird, it lost some of its power and became smaller. A phoenix is a similar creature and can be of any strength and size.

Using the same logic, you could also 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 an archangel. The game is just as "similar" as a roc and a phoenix.

P.S. I'm 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 plot than in gameplay issues?