I think the natives, if thick enough will shade out the lawn grass. The roots of the switchgrass aren't thick enough to leave a large path for water to follow if they die and decay like tree roots.
Switchgrass should be burned every other year (I'd burn it at year 1 to remove many weeds/small trees that grew from having the soil bare and disturbed). Switchgrass shouldn't be mowed when green - it's not mowed in it's native habitat, and I don't know how well it would survive.
I think that if you have a dense stand of native switchgrass, you could have lawn grass right next to it, but that's just my gut feeling. I've seen stands of switchgrass with grass paths mowed thru it, but the path didn't look like a lush lawn, more like pasture grass.
