I agree, you need a *much* bigger tank. As a general rule of thumb, you need 10 gals of water per inch of turtle shell, two 6 inch res = 120 gallons just to start. Also keep in mind that most turtle tank setup's don't fill the water right to the top of the tank. This means that your 120 gal tank is holding MUCH less than 120 gallons of water. If your shopping for a new tank, a deep and long tank is better than a tall tank.
Turtles (as you already know) are VERY dirty, more water volume (given the same water change schedual) will also help keep the water quality up and the fishes happy.
Not a chance, in my experience. Do your turtles beg for food, snap at your fingers? What do they recognize as food? If they think that most material is food (like mine), they will quickly kill the plecos, just by nipping and tearing.
pretty sure the turtles would be more aggressive in such a small tank. i had a pleco in 120G with two 5" turtles. and he did fine with them. also r.e.s are poop factories, and pleco's can produce alot of waste also. so keep that in mind.