I've never had an issue with plecos biting tankmates and leaving sores... my pleco does occasionally attach to my map turtle's shell, because it has a tendency to grow small amounts of algae, but it's never for long and it does no harm to the turtle.
My pleco has been with my turtles since I first got them about a year ago, and has never ended up with nipped fins... and Frank is right, plecos create a lot of excess bioload so be prepared for heavy, heavy filtration and frequent water changes.
I'm also curious about the other turtles... could you post pics of all five by any chance?