Wow that sounds very interesting - any pictures or more details? I don't have a lot of info on your question but perhaps you could explain a bit more what kind of parts you want to machine and what operation the mill will do.
For example do you envisage turning something then stopping and milling flats or slots on what you have turned or do you intent rotating the stock while milling it.
My first instinct would be to use mach3mill and treat the lathe spindle like a 4th axis but I may be off the mark here. Does teh C6 have an encoder for positioning on the spindle?