Emperors can be nippy, so not a good idea. What you want are the species in Hyphessobrycon. Not all of them, there are well over 200 in this genus, but several are disk-shaped and peaceful. There are several with a basic reddish/purplish background, like the Rosy Tetra (below). But research them carefully, some are real fin nippers. I include the Bleeding Heart, which was fine in my tank for many years, but others here have reported it can get nippy when confronted by an angelfish.
