Why not?
Just limit the species of shrimp and crabs.. for example a coral banded shrimp might not get along with other shrimp, calico crabs may harm slower shrimp and crabs, etc.
A 125 gallon full of sexy shrimp.. I wouldn't mind that!
On the note of crabs and shrimp attacking fish.. eh.. emeralds won't hurt much other than what's slow and sick (dragonets, pipefish, nudibranch,). Some of the larger crabs can be nasty like arrows, sally lightfoots, and some are just bulldozers such as calico, large hermits, etc..
Shrimp on the other hand other than coral banded and mantis I don't see a threat to fish. Even pistol shrimp are relatively harmless more often than not.