Below is the question I asked FFG and their answer:
Q: Is it intended that Rumor cards be used when playing the Mists of Bilehall or The Chains That Rust single Act campaigns? Obviously they could be used if playing the Mists of Bilehall and The Chains That Rust as a combined campaign.
A: It is not intended. I think rumors could be used during the Mists of Bilehall if a group wanted to try it, but The Chains That Rust would not work well as the overlord would need to discard all Act I cards immediately. In either case, the system would not be fully utilized and so would be suboptimal in its execution. Thanks, Nathan Hajek Game Developer Fantasy Flight Games