Because of their stats, some troops are more compatible than others with troop types. This is typically due to incompatible ranges or speeds. Note the adding sanc abilities or forge equipment that adds ranged or speed may change this.
You have to have all your Great Dragon armor (Kingdom City Dragon) before you can dispatch it on attack ... thats why it isn't showing up. Also, you have to have some Arerial Attack Research - not sure which level but I know you have to have some of it - sorry can't remember lvl.