Literally any army type would be fluffy for them. Every legion had the capacity to fight any type of battle. The UM in particular, they had 250,000 legionnaires, so you name it, they could field it. Siege force, void warfare, mass infantry, mass tank... Each legion was kind of known for a type of war but at the same time each legion would have fought every type of war. They even fielded Destroyers who I'd have thought might have been a bit much for the Ultramarines; there's a picture of an UM Destroyer in Tempest.
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