Actualy traveling merchants exist in both fallout 3 and fallout new vegas. As for money having weight... um no thats just not gona happen. In order for it to happen they would need to model higher denominations of currency and thats just one big bother.
Hm, i dont think its a huge bother for the developers (but im not a game designer so i guess i don't know). It shouldnt be a bother for you, cause real money has coins, and different valued dollar bills. The only hassle would possibly be exchanging them to a more valuable coin to reduce weight, and some people would want that challenge. thusly, it should be optional.
Rebels will most likely just melt down the septim IMO. the septim has a fixed cost that is a constant set by government the throughout the empire. They obviously have a government body in place for keeping it's value.
anti-imperialists governments don't have the same value set on the coins. so most likely they wouldn't use it
otherwise, the Empire could increase their supply of the septim in the empire's economy, so the septimes in anti-imperial provinces would become worthless.
a gold standard, however, keeps it's cost, and it takes much more manipulation to change the value, supply, and demand