From my experience, bettas attack anything that moves, even adult shrimp...i would be careful. Also check your gh/kh levels, a lot of people ignore this and PH swings/molting problems will eventually kill your shrimplets. I believe KH need to be around 3-6 and GH between 7-9 for RCS. Careful with the PPM too, if you can't get RO I would use distilled water. Oh and the little things are copepods (aka cyclops). Your betta prolly eats them (my guppies love them). I like to have those bc they breakdown detritus further and they also let me know how my water is, kinda like a canary...if they start disappearing then you know something might be up...