#4. "RE: Can I use a SB-800 as a Master and a SB-80DX as a slave?" In response to In response to 2
Yes! If you have the Master Speedlight set in Commander mode, the pre-flash sequence will fire the remotes in SU-4 mode before the shutter opens. The Speedlight that you use as the Commander must also be set in Manual flash mode.
You could have the Built in Speedlight (set in Manual flash mode) to trigger both the SB-700 and SB-900 (both set in SU-4 mode), the SB-600 (set in Manual mode and in the hotshoe of an SU-4), and the SB-80DX (set in Wireless Slave mode).
You could also use the SB-600 on camera set in Manual flash mode to optically trigger the other Speedlights as discribed above. Good Luck and Enjoy your Nikons!