Co2/Nitrogen
 

What mixture of co2 and nitrogen would I need for a stout and can I force carbonate with the co2/nitrogen mix and serve with co2?

As Guinness Extra Stout (and most commercially available stouts) do not use any nitrogen, I would have to argue that you do not need any nitrogen at all.

I find that nitrogen gives a wonderful creamy head, but severely affects the flavor of the brew.

To improve head retention I have used Brumore or flaked barley. Both have given excellent results without impacting the flavor.

-a.

you don't NEED nitrogen for a stout... but if you're interested in putting your stout on nitrogen, search the site for "nitrogen" -- there are several threads with good information that would be better read first-hand than summarized by me here....
or search "widget", those threads have some interesting info in them also...