I wouldn't call myself a monsterfishkeeper (yet), As I have no tanks over 30g. That doesn't mean I don't own any oddball predators!
Dwarf Puffers
Irrubesco Puffer
South American Puffer
These puffers are hunter puffers that I keep. You can keep ambush puffers that bury themselves and are a bit more monster/aggressive in a small tank as well.
You can also keep a pair of Neolamprologus Occelatus in a 10g. These guys are insanely aggressive, attacking the owners hands during aquascaping, and in the wild are known to attack crocodiles! They're pretty spiteful, and breed really easy if you're into that kind of thing.
I also keep an african butterfly fish. I had a pair but one of them died during mating (I never even saw eggs), and these fish are true mini-monsters. I've fed mine earthworms longer than his body, and live shrimp which he has grabbed from the surface of the water. He'll also hand feed if I'm patient, just like all the puffers.
There are some other oddballs for 30g and smaller, I know of some people who keep exodons in a 30long but Im not sure if that'd work out in the long run.
You definitely aren't limited though. As I've only got the space for small tanks I keep mini-monsters myself, lots of character in every one.
This is my first advice post so steady on the flaming fellas
