I've tried cukes, and my plec couldn't have cared less. That being said, some other folks on here (sweets I think) have had good success with cukes. Now I HAVE tried zucchini, which was a big hit in my tank; not only did the plec practically inhale it, but the loaches and the angels thought it was pretty yummy too. They even ate the rind, which supposedly they don't like.
It generally suggested to blanch veggies before feeding them to the fish; you can cheat by heating it in the microwave for 10-20 seconds and letting to cool before putting it in. You wil want a veggie clip or rubberband it to something in the tank as it will float otherwise.
Lastly, don't leave it in the tank too terribly long; don't want it rotting in there. I left the cucumber in for about 12-14 hours overnight. When no one touched it by morning I removed it and threw it away. The zucchini was eaten overnight.
As an aside, you might want to put the algae wafers in right after you turn off the lights for the night. Chances are your plec does a lot of his feeding then, unlike the others, and stands a better change of getting to the wafer.