I don't think we can be sure about L-brackets yet, but as D200 brackets do not fit the D300, I would be very surprised if they would fit the D700 (which is more like the D300 and takes the same vertical grip).
DX-format lenses will work fully on the D700 in "DX Crop Mode", whether they are Nikkor or another brand. This uses the central 5.1MP area of the sensor. Nikkor DX lenses will cause the camera to switch to DX Mode automatically; 3rd Party lenses should do so, but in practice some may not. You can always switch manually, but I'm not sure why someone would buy a D700 planning to use it mainly in DX Mode.