Those rocks are kind-of big -- too easy for uneaten food to get down where the corys can't get at it, and to decay into potentially toxic
nitrite bubbles. Also, corys (or any bottom feeders for that matter) prefer fine gravel or sand for the substrate. You can save the larger rocks, and scatter some of them on the surface of the finer substrate for a more diverse appearance.
Live plants only factor into the # of inches of fish per gallon in the same sense that decorations and the substrate do -- solid objects reduce the total amount of gallons of water in the tank. And you'll need a deeper substrate for the plants to root in than you would for artificial plants. Depending on how much decoration you have in there, you could potentially reduce the capacity by up to 10% (though I guess around 5% is more common). Plants more factor into appearance, and the type of fish you keep -- most like to have hiding places where they feel safe, and plants (live or artificial) provide that.