You could just install a few roof vents close to the peak but, if you really wanted to build one of those i dont see why it wouldnt work, it would do its purpose for sure, just extra unneeded work if your needs was just ventilation
edit: that one cupola probably isnt enough ventilation by the way, i would go with 3-4 roof vents spread out
if you didnt want to install more soffit vents (thats if u didnt have many) a simple way to help that out is just install couple more vents
On your full hip (4 square) roof a cupola would be perfect, just size it correctly, as you said.
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