If it was black, I would lean toward it being a pleco of some type. It would help if you could post a photo. As for the color change, fish in the fish stores won't have their true colors. They are very stressed by the overcrowding in the tank and lack of cover/hiding places. They will begin to show their true colors once they are in a normal tank and become comfortable with their surroundings.
The only loach I can think of that is considered aggressive as they get older and are harmful to corys are chinese algae eaters which are sometimes called sucking loaches. And as you mentioned, they rarely eat algae unless it is all there is to eat. They are definitely not good fish for a community tank, especially if you have slow moving, flat bodied fish.
Unfortunately, dyed tetras don't usually last long. The dying process isn't good for them so their life span is severly reduced. The fact that one has not grown may mean that it is not doing well.
