well... that is tiny compared to a lot of thing on here lol. No offense. It is a good size to start with though. Not to small so your super limited but yet not to big where you get over whelmed by it. I started with a 10 gal so, yeah.
I have no idea wat kind of fish are considered Monsters(total newbie to this whole thing)keep in mind all i keep in the goldie tank and a 10 gallon planted thats it...so i need help in this also thank you...
A smaller bichir(Polypterus) species, like the senegal bichir might work. They top out at around 12 inches in captivity. Albino senegal bichirs don't grow as big. Both are not expensive. I bought my normal for $6 and my albino for $8. Most albinos I see for around $15 in my area though. They seem to be tough fish. Interesting looking too. You might have a hard time finding tankmates that won't get eaten/attacked. Only tank mates I've been successful with are gourami. Some bichirs aren't as aggressive, it just depends on the individual. A secure lid is also important
there are plenty of options, with smaller predatory fish like Belonesox, smaller pimelodids like bumblebee, etc. There's also smaller cichlids, while not necessarily predatory, would be in a way 'monsters' due to aggression.