Hi
Some people have added shrimps to a betta and been ok, some end up as dinner! Very much depends on your betta and if he will tollerate them? Unfortunately there is no easy way to find that out without trying it.
I guess the shrimp are too small. My betta never bothered the Otto or a Siamese Algae Eater but he killed those shrimp pretty quick. He grabbed one, shook it like a dog, chomped a couple times and that was it for the shrimp. I feel bad.