I keep a few B. cupido. I would not describe them as entirely peaceful, but certainly not brutes. Mine live in a planted 55 and ignore the vegetation. They were being kept with Satanoperca acuticeps which were the same size or slightly larger. The cupids were the dominant fish. Currently, they are w/ similar sized black-banded red hooks, Geophagus steindachneri (young adults), Geophagus suranumensis, and Crenicichla marmorata (juveniles, obviously) without any problem. If putting them w/ discuss, I'd definately make sure the discus are larger.