In the PT we saw a handful of Neimodians on the bridge of their ship, a few people in the conference room and that's it.
On the Invisible Hand their is Surfer Dude Fed, Dooku and his 'captive'.
Grievous can exist in space with relative ease so walking around a ship with the life support switched off would be a doddle.
A small wing on their vaste ships is all they really need.