Yikes. Common pleco? Or does it look more like a long skinny fish with kinda dotted lines along the side (which would be a Chinese Algae Eater, probably) ? Either way, you've got to find a new home. The CAE will get large and aggressive and probably not even eat algae anymore, and the common pleco will get to about a foot and will make a horrible mess in your tank. Usually for common's we recommend at least a 75 gallon tank.
I agree, you should move those fish and it would make the cories much happier if you added a few more of their kind, like 4. But only if you get a new tank for the tiger barbs. What kind of substrate do you have?