Peaceful for the most part unless they are breeding and you have multiple males then they can be somewhat aggressive towards each other. It really comes down to the individual fish.
May be touch and go with the discus. Discus prefer different conditions than other cichlids. For one they like much warmer temps. Plus they do not like fast moving fish. They also like to be in groups. I wouldn't try it but if your discus is on it's own, you can try it but be prepared to remove the lab.