My preference is 15-18 leads per PCI-E cable, and scrypt only.  I would not run these in dual mode.  Having an inline fuse would be useful in case of mistakenly enabling dual mining mode.

As for length, I'd go up to 1 meter for flexibility. With a single cable going from 20cm and scaling up in groups of 2s or 4s to 100cm.

I'm ready to purchase enough for 50 miners right away, and likely for another 100+ miners in the near future.