If I recall, scarabs were only seen, as you explained, alone, only one. The exception to this is on the Halo 3 level "The Covenant," but this was the highest ranking situation possible, activating the rings to begin the "great journey." Now if I remember correctly, during the first battle of earth many scarabs were deployed over a large area (Mombasa) many falling victum to the strange human streets (from "turf" describtion.) It is a possibility that we will be able to mass produce these monstrous machines.
now for my opinion,
with no knowledge of how powerfull scarabs will be, it is hard to determine what role they should play in Halowars. However, scarabs should NOT have a ridiculous cost. The cost is for the actual armor, probably equal to 5-10 wraiths, the crew is most likely leckgolo, which is free. In multiplayer, the size of the map should determine the amount of scarabs allowed at one time, in campaign the priority of the mission.
the "beam" version of the hunters fuel rod seems to better fit the game, as grunt fuel rod squads would be cheaper and just as powerfull as hunters.
that pretty much sums it up.
EDIT: in responce to Ascendant Justices last responce: ever wonder what that door was in the demo? Although its probably changed, it does seem rather... convinient.