I'm having great success keeping RCS with my male betta. A healthy shrimp will be able to out swim a betta (sounds cruel letting them get chased but it's natural, in the wild shrimp are purely a cleaner and food source for bigger creatures), they are simply too quick for something like a veil tail to catch. As long as there are plenty places to hide, a decent sized population, your tank is cycled (and has been for several months) and there are no changes in water conditions, then RCS make a great addition to a good sized tank. If you are worried about losing a few shrimp maybe stick to ghost shrimp which are larger and cheaper to buy.