Cory's aren't for cleaning the gravel like most people are led to believe.
The cory's will however kick up things from time to time but it's not like what you're thinking. And yes cory's will get very stressed when not in a group of 4 or more so that's a no go, I'm sorry.
The only thing that can help clean your gravel is you, you can try doing two 25% changes during the week to help keep it clean. Usually during the week I'll use a never-seen-soap Turkey Baster to clean out their poo and then gravel twice a week which seems to help loads.
Do you have a filter?
Rembrandt II, Suiché
Leopards: 1:4 Hawkeye, Luna, Astrael, Dorian, Cullen
Gargoyle: Cloudjumper and Toothless