if youre talking about % damage reduction on the active, it should stack multiplicative. (like if garen is sitting in the area of a maokai ult)
if youre talking about the courage passive, yes it stacks. it should also work with the mastery. his w increases bonus resistances. doesnt matter from which source.
imo one of the best mid laners to play with on garen is orianna. not only do the resistances on her shield stack with his w passive (dont forget, that shield hp profit from resistances as much as regular hp which means even more tankyness), but also can she supply him with a speed boost and pull people into his spin with her ult. on top of dealing her own damage of course.