when running metal gears -- I don't bother to shim. the metal gears work good enough w/o shimming and don't require the exact shimming that the plastic gears need in order to survive.
i've never tried mixing metal/plastic. i mostly run stock -- and i like the confidence the metal gears give me. just one less thing to focus on.
running just two of four gears helps yield a smoother feel with the metal gears. over course you must put thicker fluid in. to me -- 2 gears w/ 5K feels heavier than 4 gears w/ 2K, but lighter than 4 gears w/ 3K