Super mutants could use rocks or possibly stale feces...
Come on.. Super mutants in fallout one and two are a world apart from the DC ones. Some of the http://fallout.wikia.com/wiki/Master Super mutants are dumb yes but they are not dumb enough to use rocks or feces as money. Many are smart like http://fallout.wikia.com/wiki/Marcus from fallout 2. In fallout one and two it has been proven that many west coast super mutants are just like when they were human, with all their smarts and memories intact. They used the same money as NCR, gold.
The East coast ones, have rare ones that are smart but none that are close to being "human" as the West coast master mutants. Not even Fawkes or Uncle Leo and because they are from the vault 87 FEV they will just keep getting bigger an dumber as they age till they become Behemoths. This does not happen to the West Coast FEV mutants.
The DC ones would just kill you and take what they want. They would not pay for anything. The semi-smart ones would use what the humans in the area are using for money. Till they become out of control Behemoths that is.
As for evey faction having their own currency is odd because over 200 years in that part of the usa (the West). Things have got to the point were the Big player NCR was using gold.
I can see Caps and some casinos only in the city of Vegas using Chips, but anything else like "rocks or feces" not likely.
Maybe some other form of money used by who ever is running the City of New Vegas, maybe pre-war money.
Gold should be the most valuable.