On our last trip, DH and I had two anytime FastPasses per person, per day for the entire week. There are really no restrictions other than what is stated on the FastPass (or in our case, on MDE). You don't have to use your regular passes first, and you can use them in any park. One day, we had evening FastPasses for Animal Kingdom, so we used our anytime passes for Test Track and Soarin' at Epcot earlier that day. They were not good for SDD, FOP or ToT, but I don't remember any other restrictions.