Was at my local pets at home today, and asked them about putting some of there small carbs and lobsters into my tank. I have got a tank of tetras mainly, a few guppies, all friendly fish. There advice was that "they will be fine" just asking the opinion of you guys in here, because I don't really trust there advice in the big pet stores.
your instincts are correct don't trust what they say as gospel always check up independently and yes its a no lobsters and craps of all types will catch and eat fish when able to usually at night when the fish are resting sleeping near the bottom of the tank
Also I looked into getting Red claw crabs for my tropical tank, and they actually require a brackish water setup not freshwater, there are a couple of freshwater species I believe but generally brackish is what will be required. Unlike fish the crabs can live in freshwater for a period of time. However it will severely shorten their lifespan my understanding is, and basically they need their own setup. This is on my to do list of future projects.