I feed my bettas a VERY occasional treat of cooked plain shrimp. They absolutely love it. As long as it's plain, it should be fine to feed them pieces about the size of a betta pellet. I see no problem feeding them shrimp as long as you are careful how much you feed them.
Yuzu (Yellow Dragon HMPK) / Rei (Marble Dragon HMF) / Quill (Brown Tabby and White Pancake Monster)[/FONT]