Newb flash question

Hi All,

I have a flash question I was hoping you all could help me out with.  I am experimenting with using my sb600 as a slave with my nikon d300s.  When the flash is on camera and I want to use it as fill, I use either A or S mode with shutter speed set at a minimum of 1/60 (unless there's more light) and shoot away dialing compensation as needed on the flash.  If I want the flash to be the primary light source, I set the camera to M and the max sync speed for the d300s which is 1/250 and again shoot away.

Now here's my question--with the flash off camera as a slave, does anything differ whether I want to use the flash as fill or as the primary lighting source?  I know for flash compensation I have to dial it in on the camera rather than the flash (when the flash is off camera) but other than that is there a difference?  In other words, with the flash off camera as a slave, do I still put it in A or S to use as fill and M to use it as the primary source?

One final question--is it possible using the set-up I explained (one sb-600 off camera and a d300s) to use the on camera flash as my primary light source and the off camera flash as fill?  I cannot figure out how to do this in the custom settings for the life of me.  The only thing I can think of is to set both flashes to TTL and set the camera to M and then dial back the exposure on the slave.

Any thoughts?  Thanks!

Gabriel