Sorry about the BGK and zig zag. Those are both cool fish that are easy to find. Unfortunately too big for most people.
I am not sure if the parrot or severum will pick on them, but there are lots of knives that will work. The African brown was the first one I thought of like everyone else. I think it works the best with your stock as well. There is the other brown knife (Apteronotus leptorhynchus). Similar would be the knife with many names green/glass/transparent) (Eigenmannia virescens).
Ossa knives are very pretty, but lay around like they are dead. There are also thermometer knife fish. Pelican knives stay smaller, but are hard to find and expensive. Centipede knives are also cool. I just saw nebulosa knifes for the first time they look pretty cool and stay under 8 inches.
I am sure I am forgetting a lot. Outside of the two browns I am a little unsure how the rest will hold their own with the cichlids if they decide they want to pick on them.You are kind of in limbo on your stocking.