1. The rule of thumb for focal length being equal to the diagonal of the format is based on taking lenses at infinity. Lenses used at closer than infinity (i.e., greater bellows distance) can be shorter.
2. Four versus 6 elements: I'd rather have a 4 element lens perfectly aligned than 6-elements with some elements off center. Some 4 element lenses are of low cost design; others, notably the El Nikkor 50mm, and the Schneider Comparons (no longer being manufactured) are excellent performers when used at their designed magnification ratio. I have a nagging suspicion that some of our preferences for specific lenses come down to specific examples, where the alignment is exactly right tested against other examples where the alignment is off a little.