Combining angelfshes is never easy, less when you already have an established fish that practically owns the territory, the real question is how big is your tank? An emperor angel demands a large tank, angels are aggressive towards each other in a closed space such as a tank, but if your tank is big enough to house them then it may be a matter of "welcoming them" and that's it, although it also depends of the emperor's personality, it is also possible that he becomes aggressive against lemonpeel. I suppose you want to acquire a juvenile copy.