There ARE rules that state that an adult cannot sleep in the tent of a boy that is not his or her son.
In our troop we have thought about using hammock for the older boys with @least therank of Star. Kind of make it a reward for the older boys.
As for sleeping, as digrat mentions, the rules are very strict about adults (or Scouts 18 or over, such as Venture Scouts) in a tent with a Scout--the adult must be the parent or legal guardian of the Scout in order to share a tent, be alone in a vehicle together, or any other situation where they have privacy. Two Scouts may share a tent, no problem. In fact, three in our Troop often share a tent so one can carry the tent, another the poles and the third the rainfly.
The BSA doesn't address hammock camping situations such as hanging two hammocks under the same tarp, but I would say the adult should NOT be in a hammock like that unless the Scout is their son or the legal guardian relationship exists, even though a tarp isn't exactly private. You just don't need the trouble that might invite. The rules do as much to protect innocent adults as it does the boys.
-- Tim Taylor