You got US Composites epoxy there. If you were to go to their website, like I just did, you would find all the pertinent information listed. They make two resins and 4 different hardeners. You have the #635 thin resin and the #556, 2-1 slow hardener and yes they are compatible. So is the 3-1 medium hardener with the #635
You can use three of the hardeners with that resin.