I have 4 different sizes of African brown knife fish (came as a set), an 8" bishir,, and a 6" kissing gourami that stays in the top half.
People have warned me that those knifes can turn aggressive but ATM they are very close and school together.
My tank is a 150. I have it hooked up to a Marineland canister that turns over 530 gal an hour. I have plants and room for everyone to have their own space. So far so good. I will rehome to my lfs anyone who becomes a problem.
Those leaf fish are gorgeous but I've heard they eat anything that can fit in its mouth. The knife fish do get big and so does a bichir. I've been told figure 50 gal per knife and bichir. I'd say just research everything you can (heck I might even send you an extra knife sometime lol) and make your habitat fit the fish, not the other way around.
The black ghost knife, I have heard, gets bigger than brown ones. Long ago I had a Senegal bichir who ate my black ghost knife. Took me a long time to trust an eel again
. My striped one I have now leaves everyone alone but I suspect he may have killed a little cory I had. I bought a monster of a green one not long ago and the eel does not mess with it.