even if it doesn't eat it, it could severly damage it! remember, anything in ur turtle's tank has potential to be a meal! that being said if you have enough hiding spots for the pleco im sure it would reduce the chance of it being food!
My pleco was twice as big as my turts and they still attacked it. So, the pleco now resides in the snail breeding tank. It annoyed me, but I knew going into it, that he might be food. I couldn't watch them tear chunks out of him though.
My pleco is about two to three times the size of my turtle. He hasn't bothered my pleco yet and it's been about three to four months since I've had it but that doesn't mean later on he won't. But I think Michelangelo realizes that the Pleco is a good thingy cause it helps him keep his tank clean by eating all that algae.
You should never feed goldfish. Rosy red minnows or feeder guppies can be given occasionally as feeders. Goldfish are considered like junk food. They are fatty and tend to carry parasites that could harm your turtle.