Hi SailfinGirl
Emperor tetras are amazingly beautifull fish and an excellent choice for any community tank. They are a schooling fish so get at least 5 or preferably more.
I've got a couple in my tank and they have never been nippy towards any of my fish (guppies included). They do like to hide between the stems / leaves of tall plants and occasionally hang motionless in the water with their heads slightly lower than their tails, don't worry about this it is perfectly normal.
Good luck