I've personally seen Ghost Shrimp nab a feeder Guppy at a LFS. Quick little things but then again the poor shrimp was probably starved. Personally I have 5 of them in a 10g with 6 week old Guppy fry and haven't had a problem. But the possibility is always there, like being hit by falling cars.
I wouldn't worry about shrimp and bio-load. If anything they help keep the tank clean by picking up uneaten food and such. My Ghost's keep my moss nice and clean compared to what it was before.