Have you checked the Ph levels? If the Ph is too low or high, this could cause stress and disease. I bought a Ph tester from Wal-mart and is a great resource. How big is your tank? Make sure you put aquarium salt in your tank (based on size) as tetra's and the plecko need this to slime their coats.
How is the air circulation and filtration system? Once a week, I put a teaspoon of Melafix along with stress enzyme in my tank.
I've heard that you can place one betta in a community tank but have never thought of experimenting with this.
Plecko's are very hardy and spiney fish. Mine's been in my tank for 7+ years and doesn't do what he's suppose to do and clean up messes.
Hope you can come to the root of the problem Dracen.