The .5 is too small for the cory, and I wouldn't keep one in the 2.5 either. They prefer more of their own kind, and a 2.5 is too small for a school of cory cats. Not sure about cherry shrimp, as I have not kept them. My concern with them and a betta is their red coloring, small size, and a betta's predatory nature.
In my experience its down to the individual betta in whether or not RCS will become a snack for them. Baby shrimp will most certainly be eaten, but typically a few will survive. This is actually good as RCS reproduce like rabbits.
I actually really like nerite snails and never had a problem with their eggs, but my fish could have been eating them before I noticed they were there.