Ok lets start here....Crushers basically punch spearers are sharp and stab your fish, if I have tiny fish like blennies and gobies and basslet, they would either not be considered a Threat if the shrimp is well fed or they would avoid the shrimp
You have a 20G long tank, thats roughly what 30 inches? Now how does anything get away, or even stay away from something thats trying to murder it? there simply isn't enough room.
Whether they are a threat or not, does not matter. Mantis shrimp kill to eat, It's called nature and regardless if you feed it, it's in it's nature to kill.