Yes, bettas will eat shrimp but likely won't be able to handle an adult shrimp at 3/4".
Most fish (including danios) will eat the young shrimp if they can but that helps keep the population in check. A 20gal tank with lots of hiding places as should be able to sustain a healthy shrimp population. I would be sure to keep some sort of moss (like java moss that I include) in a mass/mat large enough to keep fish out of (say a softball size) so the young have a safe area.
Hope this helps!
-Bill
Most fish (including danios) will eat the young shrimp if they can but that helps keep the population in check. A 20gal tank with lots of hiding places as should be able to sustain a healthy shrimp population. I would be sure to keep some sort of moss (like java moss that I include) in a mass/mat large enough to keep fish out of (say a softball size) so the young have a safe area.
Hope this helps!
-Bill