I think there won't be any imbalance. Either the dwarves will be very strong (strength + magic) and expensive, but with a small growth rate, or they will be weak and cheap but with a large growth rate. There's no third option in the game (the golden mean doesn't count).
I know that with innate resistance to fire magic, mages will have a harder time attacking (there's a player with Chaos magic that gets stronger with each level) using fire magic.
But overall, this is better than the swamp dwellers' hero in Heroes 3 (Hydra).