You can put shrimp in a sorority, but I wouldn't recommend RCS. They are small and expensive so if any of your girls decide they want a snack, they're not really big enough to put up any sort of a struggle to stop it. Also, both the males and females are colored, so there's not really anything you can do about them reproducing, but if they did your bettas would more than likely eat them anyways. You usually don't have to worry about feeding shrimp in a planted tank, they'll eat algae and junk off of pretty much any plant, but if you did want to, you can just sink 1 or 2 pellets (or flakes or whatever) to the bottom while you are feeding the bettas and the shrimp will find it soon enough. If you do get shrimp, I recommend glass shrimp because they're a little bigger, tougher and cheap so if your bettas eat them its not a super big deal. I have about twenty ghost shrimp in my 29 gallon sorority and aside from a few bit off antennae they're fine.