What ever you do if you put a 19" tire that is .2 taller than a 20 " tire it will look WRONG!
The front tire profile with be taller (thicker) than the rear.
I CAN"T tell you how may times the "TIRE" guy SCREWS THAT UP!
Rule #1
if the front rim is 1" smaller I use a tire that is at least 1.25 to 1.75 shorter than the rear.