Yeah, it's totally dependent on the betta's personality. I've heard of bettas never touching a hair on a shrimp or snail's head, and bettas that will kill on sight any living intruder. And I've heard of bettas that were fine with their tank buddies for months and then one day just decided to bite their heads off.
I suggest you include a whole bunch of java moss and tiny hiding areas (cholla wood would be good) in the tank to up the odds of shrimp survival. And maybe breed them before you put them in with the betta if you want to keep owning RCS
