I put two coats of the clear over the Rustoleum Professional Epoxy Shield when I did my floor. Been 2 1/2 years and still no stains, everything wipes up very easily. If you do apply the clear I would suggest adding Shark Grip or some type of anit-slip. Without it is a sheet of ice!
One more comment, you mentioned "hot tire p/u" as one of the reasons you want to apply the clear. I'm no expert but I'm pretty sure preventing hot tire p/u has more to do with the prep work on the floor than applying a clear coat. The toughest clear coat in the world will be worthless if your "base coat" of epoxy did not bond properly to the concrete. Just my $.02