The plural is pegasi, yes. XD

Plegia's army is mostly compromised of Barbarians, Dark Mages, Wyvern Riders, Myrmidons, and Thieves, but since it doesn't say anything about any Pegasi Riders, then there can be.

I can't wait to bring in Lumara... *dances* You know in Fire Emblem Awakening where random people jump into the battles, like Sully, Virion, Ricken, etc? She's gonna make an appearance like that.