They will grow large enough to threaten shrimp, though.
The small puffers or rope fish will definitely eat some of the other inhabitants.
Also I misread the OP. Thought it was a 38g tank. So most mormyids would probably be big for it, yes.
Are you thinking of Kryptopterus minor? I mean, they max out at 3" and from what I've known...wouldn't hurt anything other than flake food...is he raising brine shrimp? Are you confusing it for K. bicirrhis? They get around 6" as adults...Stop stealing my thunder...let me have this. BIRD CALL! KAKAW!