If different varieties remain separated by 2 feet when fully grown, chances of a natural cross will be fairly low. If they grow into each other, as plants often do later in the season, you will increase the chances of a natural cross. But even if you get a natural cross, you'd then have to harvest seeds from that particular crossed pod and wait for next year to see the results. To reduce you chances of a natural cross, only take seeds from pods that are most distant from others varieties - some say use the pods near the center of the plant.
My plants end up touching each other toward the end of the season. I try to group similar varieties together but they do mix a bit. In 6 years, I've had one natural cross which in which I preserved the seed and kept it going.
I'm relatively new to intentional crossing, but I've only had about a 25% success rate (many flowers fail to pod).