You could try an Amano Shrimp - they're a bit bigger than cherry/ghost shrimp so they won't be as easily snacked on. I have my betta boy in a 5.5g with an Amano and after a few days of my boy chasing the shrimp around they mostly just ignore each other
I started with some Cherry Shrimp in a 5 gallon with a male betta. He's a fairly laid back boy that's only ever flared once, and that was not even a full flare, just a slight raising of his gills. He's a long finned boy too, so it's harder for him to chase the speedy little shrimps.
I started with 5 RCS, them added 5 more about a month later. I did once see one of the shrimps missing most of the legs on one side and I'm not sure it was the betta, but he seems to like stalking them even if he can't really catch them, so it's a high possibility.
I have a slight algae issue in another 5 gallon, but I thought the Cherry Shrimp might be at a disadvantage because the betta boy in this tank is much more aggressive. He's also a plakat so he's fast! For this tank I got a single Amano shrimp, which I rarely see because he's sooooooo shy and because he knows that his tank mate is a predator! I have thinned 4 of the Cherry Shrimps from the original tank (3 males/1 female) and added them to this tank as well in hopes of making Manny the Amano feel more comfortable. Manny is doing ok but I don't know about the Cherries because I never see them!
I LOVE watching the shrimps do their thing but it is a risk.
Oh, and I tried the nerite ansil thing first but I only really saw them after they died! I dunno what happened but both died not even a month after I got them.