Mine get along just fine. There are 5 ghost shrimp in my 10g with my male betta. About as many in the 18g with the female betta. The ten has almost no coverage because I'm growing all the plants from bulbs and cuttings, the 18 has lots. Neither tank has any problem with betta-shrimp aggression.
Bettas and ghost shrimp should get along, though, in some cases the bettas may eat the shrimp. It really all depends on the personalties of the bettas you have.