I am against dwarf shrimp like RCS in with Bettas; too stressful for the shrimp. But I do understand their appeal. Their bioload is so small that you can have 10 shrimp per gallon and the more you have the better for the shrimp.
I agree with Kittenfish about waiting. I suggest waiting at least a couple of months beyond cycling to add them. This gives the tank time to stabilize and develop the micronutrients/biofilm on which RCS feed. They are not "cleaner" shrimp but have very specific feeding needs.
PS: I had shrimp in with Bettas forever and have noted a tremendous change in behavior and fewer deaths once the Betta was removed from the equation.