Cruel?
No, especially not as a baby. As a full grown adult, you will probobly want to move him up to a bigger aquarium, at least a 55. But for a while, he will be fine. But they grow like weeds.......
Will he eat a clean up crew?
Very possible. But the good thing is, if he is not cramped and well fed, he may leave them alone. And a lot of times he may only eat them as an occasional snack, so replacement of clean up crew is an option. But if he is well fed, it is a good possibility that he will leave the clean up crew alone, for the most part............