I guess nothing can be done about the pleco. I'm really surprised it hasn't eaten the tetras. If you can find someone with a large tank keeping Cichlids, etc it would be awesome to find him a new home.
If your going to get cories makes sure you don't get them until your tank is cycled (completely).. this means a month or more. Once again DON'T GET THEM UNTIL YOU ARE CYCLED. This is because cories are scaleless fish and are extremely sensitive to the water conditions. Putting a cory in the tank while cycling would be very very bad for the cories.
I'd also suggest getting more tetras. If one dies that is a cue to go get more. But in all honesty if 3 of your tetras died I would be looking into finding out why.