I agree, because the Rhea are 'fallen warriors' with their wings too injured to be able to fly. But their tenacity motivates them enough to take up their weapons and continue fighting with their injuries
The next advancement however, is pretty hard to think of due to the race's limitations from wearing heavy armor.
For the 'magic' users or general healers of the faction; I am thinking of potion pitching units, which can change to better healing unit (herbalist maybe) or as an alchemist, throwing fire bombs from the air instead.
Edit: Also, if you want the portraits as well, just tell me