Picassos are known to be highly aggressive, I don't know that you will ever have harmony. IF your profile is current, I would be thinking that your crabs and snails are gonna disappear soon as well as possibly the other fish.....maybe redoing the rock formation will help, but normally that is the fix when the "original" fish in the tank are being too aggressive, not the add on. Hopefully the pvc
will help, but even though that may save the life of the niger, it could turn into a situation where you never see it out anymore as it is being too terrorized. Time will tell.