Your birds are going to destroy pretty much any vegetation in no time flat, so you might as well start with bare ground lol.I have dirt in my coop where i built it so do i need to add anything to it
I'd leave the mulch. I actually add stuff to my run all the time--straw, leaves, horse manure, wood chips/shavings, etc. They'll arrange things like they want, and chose their own area for a dust bath or sunbathing site. Mulch encourages insect population and gives the birds something to do all day.