Chinese algae eaters are scavengers. They prefer the algae when they are younger, but as they grow, they like a meatier diet. I'm not sure about the terrapin pellets, but the bottom feeder tablets work great. Algae wafers are also good. Make sure you have plenty of food for the algae eater and at least two caves/holes for him to hide in. If he feels threatened at all, he may show aggression to the other fish. Also, if he gets hungry, he may go for the other fish. If you see him starting to chase the other fish around, increase the bottom feeder tablets and put another cave in for him to hide in.
CAE's do best with at least 20 gallons for one - and if you have two, then 40 gallons or more. I have three for my pond.
As for feeding cucumbers slowing down the algae eating, if you notice that the pleco is not eating the algae, then you can reduce the amount of cucumber you feed until the pleco is eating the algae again. Or just feed the cucumber every once in a while as a treat.