I would put the skimmer in the right part, as we see it in the pic, of the 20l, then a baffle say 8" high or so. This will give your skimmer the constant water level it needs to operate properly. It will also lessen the chance of the skimmer taking out the pods and other good things that can come from the fuge. With the skimmer in the same area as the return, the changing water level will cause it to need to be constantly adjusted. I would then use the 10g with no baffles for your return and give you that much more float room in your top off. Would also give you a good place for reactors and what not.