Or you could simply make it Orcs, Dwarves, Maghar, and Black Iron, along with Goblins and Gnomes. Give Orcs Iron Horde designs, and give Dwarves Dwarven designs found in their Siege tanks, Mole Machines, and Gyrocopters.
If you want to restrict it to small races, Goblins, Gnomes, Junker Gnomes, and Vulpeira work fine. Vulpeira especially would solve any issues with racial popularity.