To my best knowledge GK's will we anything smaller then there mouths. So if your going to get tank-mate for it get something that it can not eat. And make sure your tank is big enough for both fish. 150 gallon of something close to it so the GF doesn't feel threaten or he will kill anything or possibility die of stress himself.
black ghost knife fish gets nutrients and energy from the food it eat
No.
The maximum length of the black ghost knifefish is 40 centimetres
they can be 49cm.
No.
Yes it can
Black Ghost Knife Fish are semi-agressive. They should be fine though. The angelfish should be fine.
fish about 5in or 4in lowest
Yes if its the only one?
Depends what store!
Black moors are a variety of goldfish which are egg layers so they can't get pregnant.
The fish must be small enough to be eaten by the knife. There are also small shrimp (ghost shrimp) which some small knifes will find easier to catch than the faster guppies. As the Knife grows so will it's prey.