There's nothing a general can do that a field marshal cannot do. Field marshals can command armies, and those armies can belong to an army group commanded by another field marshal. So yes.Can a Field Marshal command five other field marshals?
If all of the above is correct, you can promote when you have all general traits you want, including terrain.
You don't have to wait that long. Field marshals can buy general traits. Remember the rule: field marshals can do everything generals can do, and then some.