CRS usually means Crystal Red Shrimp, Caridina
RCS usually means Red Cherry Shrimp, Neocaridina
They won't interbreed but they prefer different parameters. I've heard of people keeping them together, but it isn't ideal for breeding.
I've seen many mixups of the abbreviations from online sellers and it's understandable, personally I try to stick to spelling out the whole name to avoid confusion, but I do slip up. Ideally, everyone would use the scientific names to be absolutely certain, but it does get to be a mouthful eventually