20-lb dumbbells are fine for Platemates. In fact, they would also fit onto 15-lb'ers as well, not to mention heavier dumbbells.
I personally think Platemates are an excellent value; I have sets that increase each dumbbell's weightload by 2.5 lbs, and sets that increase each dumbbell's weightload by 1.25 lbs. For those who want to build strength slowly, safely and incrementally, Platemates are the best route. With the help of Platemates I have been able to build up to dumbbell weightloads of 26.5 lbs per dumbbell for my upper body work; I could never have done that trying to vault from 15 to 20 lbs, or 20 to 25 pounds.
A-Jock