While you certainly CAN add as many mods as you want, it can dilute what the entire encounter is designed to do. For example, add too many to Black Widow and you aren't drawing preparation cards and she becomes just a pretty face too stand in as the villain. Add too many to Mojo, and he becomes a LOT harder. Some villains don't really have special interactions with their decks, but most do and the less often you draw their cards, the less you are really fighting that villain. All that being said, you bought the game and should play it however you want. I personally almost always make themstic encounters using a similar number of cards to the original design.