There is no problem if you install first GWX or SH3Cmd, just make sure you're using the extra SH3Cmd files for GWX, otherwise you might experience problems.
As far as the realism settings are concerned yes there are the same options. GWX is adjustable to all tastes. Usually the DDs are more accurate and more deadly but if you learn to adjust your tactics and play in a clever way you can survive longer. Give it a try it's really good
