Just to clarify about the RB rule, I'm mainly focusing on not having any WR in the RB part of the depth chart. If the game engine allows a WR to run the ball even though he's not listed as a RB, not a whole lot I can do about that.
I am saying you/your but I'm addressing this to the whole league. If you have injury problems at RB and find yourself short, you'll have to do one of three things. You can do what Smirt did and trade for a RB, you can use a FB/TE to run the ball, or you can take a WR and switch his position to RB and use him.
The rule is simply a RB must carry the ball, there are no minimum weight they must be.