Line a 9x9 baking pan with wax or parchment paper and set aside. Dump cereal into a very large mixing bowl, mixing it together if using different cereals, and set aside. Combine brown rice syrup with optional peanut butter in a medium saucepan and warm over medium heat, stirring constantly, until it becomes liquid-like and is no longer thick. Once it's liquid, dump HALF of it on the cereal and set the pot down in a safe place. Using a spatula stir the cereal gently, trying to coat all cereal with a little bit of the mixture. Add the remaining mixture as needed to make sure that all pieces are lightly coated but not well coated (or they will be mushy). Transfer cereal to the 9x9 pan and using a spatula, pack it down tightly and evenly. Let the mixture set out and dry for at least 4 hours before trying to cut blocks or bars.