Chili rasbora/mosquito rasbora (Boraras brigittae) would be great for a 5 gallon. They're VERY small. They are small enough to be no threat to shrimp. They aren't terribly active (which is another reason why they're a great choice for a 5), but they aren't shy either and you'll see a lot of them.
It's quite possible for an adult betta to eat RCS, and many will. If said betta isn't large enough to eat adult RCS at the time of purchase, it's only a matter of time- he'll grow. Any betta you buy is a threat to baby shrimp so if you would like for them to reproduce (they'll breed quickly if given the chance), I would skip the betta for sure.
If you can find them, there are several other variants of Neocaridina heteropoda- snowball, blue pearl, yellow come to mind. Do not mix different color variants.
In a 5 gallon, you could keep 8-10 Boraras brigittae or Boraras merah, and you could start with a small colony of RCS or other variant, say 10-15. You'll have more in no time. LOL.
If you want to do something really unique, check out some of the smaller wild Bettas. You'll rarely ever find any at your lfs so you'll almost certainly need to order them online. Smaller species like Betta rutilans can be housed in pairs in a 5 gallon for life.