Did you ever stop to think that you're taking the word "adjacent" too literally? Why would the 2x3 patch allow for better intercross potential than the large?

It seems a bit backward to design a medium sized option with more pros than the large, since gardening's time sink is largely only desirable to get intercrosses in the first place. By continuing to limit yourselves to grids with 3 kinds of plants only you're providing great numbers for a control test of those 3 plants, but ultimately limiting to any number of further potential intercrosses that might occur if "adjacent" is to be taken in a less literal term.