Here is a great video from Earthbox about combination boxes.
I learned that you can grow sweet peppers and hot peppers together without worrying that your sweet peppers might turn hot. I have always taken great care with separating them, and it’s good to know I don’t have to worry about that.
Hot peppers and sweet peppers can indeed cross pollinate, but it will affect plants grown from those seeds. So don’t save seeds if you mix peppers. Otherwise go ahead and mix and match as you please.