Your betta well eat the shrimp, that's why they recommend not doing it. Male bettas can be tough, tho I have mine in the live-bearer tank (mostly platys). He's a pretty mellow guy tho, and doesn't bother the lone guppy (also a mellow guy).
What size tank? bettas enjoy having the place to themselves. If you really want shrimp, go with a small species of fish that wouldn't bother them like galaxy rasboras or guppies... Most fish see shrimp as a food source.