what jessica said is waht i was worried about with my own...
how ever i still put my two western painteds in a 55gal with my 12" albino plec. i have been keeping an eye on them and they dont bother eachother and they have shared that tank for 2-3months.the turts were 2-3 inch now 4-5inch...so i think it should be ok as long as you keep an eye on them if a problem starts be ready and willing to put them in their on setup...
My monster matamata lives with 3 plecos. ok, they're huge..but I find no one bothers plecos. Say go for it.. the plecos won't hurt the turtles, in any case.
Female Map turtles have jaws designed to crush things. Their main diet is clams, snails and crayfish. I'm pretty sure they could easily crush through a plecos armor. Like everyone said, keep an eye on your turtles. Someday in the future they will be eyeing your pleco with interest.