I say have a list of characters but have an "Other" option in case we miss someone.
Sounds like a good schedule to run on it.
See I am the type of person who'd run them as two seperate contests but run them at the same time. Basically have the Buffyvese contest as it's own contest with it's first, second and third place winners and the Crossover one with it's own winners. I'll leave that decision up to you, since you're probably going to be doing the hard work with the graphics challenge.
I am going to ask people to tell me what they'd like for a fic challenge. Once a month, every two weeks, etc.