RTF is a global modification that replaces all towns in the game.

Could I have both options?
Both Valkyrie and the Ghost.

I'm not sure about that. Valkyrie is a large and fast unit, like a giant fly. I wouldn't want units on adjacent tiers to perform the same role on the battlefield.

I don't want to place them on different tiers – there aren't enough models, and I won't be able to dress them in different armor.