In regards to #1: Depends on the slots on your motherboard. Mine has 3 16x slots and 3 1x slots so i would go with 3 1x to 16x and 3 16x to 16x. There's quite a bit of flexibility there( e.g you can use 1x to 16x risers in a 16x slot). I just like everything to match properly
![Undecided](https://bitcointalk.org/Smileys/default/undecided.gif)
Length of cable depends how/where you intend to mount the cards. I prefer to go with longer ones as too much cable can be worked with, not enough is a problem.
Also if your planning on running 4+ cards i recommend getting the powered risers (with a molex plug) to avoid burning out pci bus etc.