The problem really isn't the bettas them selves, but the ghost shrimp already occupying the tank. The betta may eat the shrimp since they are carnivores and if the shrimp don't have enough hiding places. RCS are much weaker than GS and so much more smaller. It could work but it would need to be tested out and have a back up plan.
Your tank sounds nice though...
Yeah well my shrimps aren't ghost shrimps, they're Red Cherry shrimps. My buddy with the Betta actually had some ghosts, they dissappeared with the betta, and I wouldn't recommend them. They don't seem to have a taste for the cherry's for some reason, at least not in my experience.
Thanks for the compliment, it's a fully stocked 15 gallon which will be upgraded to a 20 shortly. In addition to the 3 females bettas (which get along really well, the newer two even hang out together at times) , I have a school (5) of glo fish, 6 Neon Tetras, 4 Otos, the shrimps and an apple snail. Overall it's a pretty active tank, well planted with excellent filtration (2 Aqueon 10s... can't have too much filtration).
My stocking rate is 100% on the nose (assuming adult sizes, which most aren't) on AqAdvisors calculator... I know it sounds like a lot of fish for the size, just wanted to put it out there before I got any naysayers :D )
In any case, with the shrimp, I highly recommend having a thick plant they can hide out in that isn't easily accessible to the other fish. The Hornwort worked really well for me, but i'm sure Java Moss and other plants would work just as well.