Fire shrimp, like all shrimp, hermits, crabs, and other invertebrates, are a lot like fish. If you don't overstock and each invertebrate has adequate space, they will only bother each other minimally. Like fish in a properly populated tank they will butt heads slightly, occasionally resulting in a lost leg at worst which grows back with a molt. If, however, you overstock, they will kill each other until they all have the space they need.
Basically, if you stock well, they will be fine. If this is for the 29 I would only add 1 fire shrimp, as my 2 in a 90 still fight sometimes(one just lost a leg). You could and probably should add a couple skunk shrimp that will keep your fish free of external parasites.
Good luck. I hope this helps
LCP