according to your newer thread, your baby black is currently stressing out from being in a new environment. i would not add tank mates now, but do add some floating plants or similar, and reduce lighting for it to feel more secure. do pay close attention to it and ask questions here if you suspect something is wrong. and do more research on this forum about this fish keeping - there's ton of information available here.
congrats on the new black. any pics?
come back and ask for its possible tank mates when it's > 10" in size.
good luck