The ADF's will likely eat the shrimp if the betta doesn't.
I was always too scared to try RCS because my 10 ghost shrimp a few years back were gone within a week of adding a betta. I tried some ghosts with my current ADF's just to try it out and those guys ate them as well, faster than the Betta's did back in the day.
RCS are even smaller than ghosts so no, in a tank with both those species I wouldn't risk it.
Plus bioload is wayy too high already. An ADF should have 5 gallons to itself though 2.5 is doable, and 2.5 is also the bare minimum for a betta if you don't want to do multiple w/c's a week basically you should have 7.5g minimum right now to have a healthy system