I recently had a strange experience with my betta + ghost shrimps. For 6+ months, the betta ignored the ghost shrimps I had in there. I had cherry shrimp in there for a while that the betta did attack once (so they were promptly relocated). However, it seemed to me that the ghost shrimp were too big for the betta to attack. This past week, however, while one of the ghost shrimp was molting, my betta decided to nip at it and tore a hole in the ghost shrimp. He proceeded to slowly eat the rest of the shrimp and by the end of the day, the shrimp was gone (picked off by others in the tanks like cories). This was when the ghost shrimp was big too - 1.5+ inches. So you never know.