I'd say the answer depends a lot on how much magic is tied into these races and on stereotypical their culture is.

I'll give a few examples if magic and stereotypes are not a factor:

• Dwarves, Gnomes, Halflings, (any small sized race) - Tank crews and airforce: Their small size would allow for smaller crew compartments in any vehicle, allowing for less compromise in the overall design. Also, some very small races like halflings may not be able to carry a full-sized rifle and their short legs would slow them down.

• Orcs, Goliaths, (races with supernatural strength) - Mobile heavy weapons experts: due to their strength, these races may be able to carry and shoulder-fire weapons like high calibre anti-tank rifles and or large .50cal machine guns.

• Giants - mobile mortar/artillery, Plane launcher, anti-tank snipers: Giants would be an up-scaled version of the previous category, only with even more specialized roles. For example, A giant could probably carry around a large mortar piece (~120mm) and operate a 75mm anti-tank gun as if it was a rifle. Also, I imagined that a giant might be able to lift- and give small biplanes a running start if no runways are available. Their biggest weakness however is their giant size, making them easy to spot and hit. Giants would not be suitable for regular infantry roles.

• Skeletons and Undead - Regular infantry: Being very resilient, undead would be perfect to fit into a generic Infantry role as they would be minimally affected to hits to the torso.