The only reason to have mixed containers is if you are using them to quickly dump your inventory, implying that it's not connected to the belt network, or that it is connected to some kind of automatic sorting system using smart splitters. Otherwise, for any container that is actively being used as a buffer within your factory, there is is absolutely no reason at all to have any kind of mixing. It not sane, and create a whole lot of problems which you will, without question, have to go back and correct latter. Mixed belts and containers can, and more often then not, will bring your entire factory to a standstill. Just don't do it.
